What is puzzling me about your model is just how you see this working.
How do planetary governments interfere with the freedom to travel, barring some particular local laws? Canonically, worlds can make their own laws restricting people coming in and people leaving (Exit Visa!), and for my part I see no reason to interfere with this.
How does the Imperium function if world governments are unable to enforce their laws on anyone who has ever served in any Imperial service, or been born on a starship, and so on? Or is this not how you intend it to work?
My biggest issue with the "Imperial Culture" is that there is no mechanism for propagating it. You have to be surrounded my people who have a culture to interact with it. You need someone to raise the person in the traditions of the people and have a medium (ie society) to practice it, have positive/negative reinforcement in real time, and even have the means for seeking approval or rebelling against cultural mores. You can't do that with only ONE person.
Now, take an enclave for example. If it enjoys the same features as a diplomatic facility - its soil is technically that of the Imperial home world. Using the formula I found at this website:
http://www.had2know.com/academics/hexagon-measurement-calculator.html
A 10 km hex will have an area of approximately 86.60254 square kilometers. How many people can you fit into such an enclave? If Belgium's population of 340 people per square kilometer is anything to emulate, we're looking at a potential enclave population of around 29,000 people.
Why did I pick that number? T5 indicates that a noble's single HEX is a hex with a diameter of 10 km. A higher noble gains MORE land. And if I understand what Marc wrote in his book correctly, a Baronet has his land grant plus the land grant of his knight hood. A Count has the land grant of all of the previous grants from previous social status holdings in addition to his Count holdings. Half of the holdings are on the main world, half is on a world within the system that isn't the main world.
In all? THAT is a lot of land, and a lot of people.
Now, can you require an ambassador to secure an exit visa? Can you pen in the only representative of the Emperor (A knight for instance) on a world he's been granted a knighthood on? I suspect not. Can he live his life in an area that does not have the main world's culture constantly surrounding him if he has his own enclave surrounding him instead?
Now, some worlds MIGHT want to build their own "extrality" wall around the Noble's home, and they darn better declare the airspace above his home restricted. In my Traveller Universe, the death of a noble requires that Imperium investigate it. That is a "NO NO" to have a simple killing of a Noble - it may be a sign of impending unrest etc.
Now, about those Imperials who gained it through service to the Emperor. As such, they could apply for a position within an Imperial Enclave or perhaps at a Starport. They can also simply live on the main world of their birth with their dual citizenship. As with ALL dual citizenship situations, depending on who wants jurisdiction over the victim who was a citizen of the Imperium, or who wants jurisdiction over a criminal who is a citizen of the Imperium - makes for a potential sticky issue. As such, it offers a potential for the GM to have a Noble player character be required to investigate such a situation and make recommendations based on their investigation. He might suggest "throw him to the wolves" and let the local laws take the guy down. Or he might suggest "Remand him to an Imperial Court" etc.
So, how is this any different than a starport? Simple enough question, and deserves an answer...
the NOBLE whose land it is, is responsible. He's a representative of the Crown to the locals, and he is also a representative of the locals to the Imperial government (which is likely subsector in nature).
NOW the nobles have real power. They can import technology to their enclave and not worry about whether the local government bans it or not. They can import ideas, entertainment media, etc - that the local government has NO say - any more than a nation has any say over what goes on in a Diplomats home or the embassy grounds proper. If a Westerner wants to drink alcohol in an American Embassy in a region that forbids alcohol consumption on religious grounds, the American embassy can simply ignore the issue and keep it behind walls...
In the end? Each GM can set up his own Universe as he sees fit. My problem is that I see ZERO cohesion with the background as written in official material - that keeps the nobles squarely in the camp of "Imperial Culture".
As I mentioned earlier...
Culture isn't about foods (although that is part of it). It isn't about the language (although that can be a part about it). It isn't even just the religion or entertainment or anything else that people will point to and mistakenly say "that is culture". Culture is the means by which it instills values in its youngest generation in preparation for when its older generation dies off - it is a means of continuity for the state/people/religion etc. Movies that we watch - pay attention to who the Villans are. Their behavior shows what the culture says is "Wrong". Pay attention to what the hero does to counter the bad... that tends to be what is deemed to be not only right, but necessary to counter the bad. Some entertainment blurs the lines between the two, and in some instances, the anti-hero shows what needs to be done. Honor is defined by the entertainment or by the Aesop's fables kind of things. Truisms such as "The early bird gets the worm" also show some of the rights/wrongs of what the culture wants to instill in its young.
Then we have the fascination with the lifestyles of the rich and famous or those who openly break the law. The question becomes one of "is the line between right and wrong starting to blur at this point?" or perhaps "do people want to break past the inhibitions that culture imposes"? Those are questions perhaps for anthropologists and sociologists.
Anyhow. I know that what bothers me about this isn't necessarily something that bothers other people. But - its kind of funny how Traveller started me down the road in College, of looking into Physics, astrophysics, and trying to apply my courses in sociology, psychology, economics etc - to the universe to make sense of it. That of course, was back in 1978 - I never really got TOO involved with MEGATRAVELLER (was too busy working in a factory and going to college). When TNE came out, we tried it, sorta had fun with it, but liked GURPS better. Did a GURPS/TRAVELLER fusion before GURPS TRAVELLER came out (using CT ship rules for ships and stuff, but GURPS rules for characters and combats). Then we tried T4 and HATED it. Never tried any of the Traveller incarnations again until T5 came out when I purchased two books and gave one to a friend. We were NOT happy with the combat rules and ditched it
but, for now? We want there to be more detail behind the Nobility of the Imperium and how they are part of it, how they interact with the universe, and how it kicks them in the teeth from time to time.
Now if I could only get over the disconnect between the T5 planetary generation rules and the default setting for Spinward Marches. Case in point? They're using the stats generated for Strouden using the older SCOUTS rules or MEGATRAVELLER rules where Strouden has zero Gas Giants. Um... what? At least they changed the primary star from an M class star to a G class star - but to make T5's SPINWARD MARCHES truly a T5 product, they need to start from more or less scratch instead of using the PGA setting from MEGATRAVELLER era
But, again, it can remain the way it is, and I can change it for my Traveller Universe - that's the joy of IMTU...