No, if known lanes are designated for shipments, those lanes can be easily monitored and ships that approach can be intercepted. If ships aren't available for interception, the thieves can still be easily tracked if they try to haul it away. Where are they going to go?...
Oh, a whole lotta places.
First, to quote Douglas Adams, "Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. ..." Again, I've only got the local belt as an example, they could be closer in and denser - or they could be much farther out and not as dense. If the decent-size bodies are up to a couple light-seconds apart, and the belt could be up to 8 or 9 AUs in diameter, you could be dealing with anything from several light-seconds to a light minute or more between the mine and your target processor. Either they're posting the smelters close enough to keep everything controllable - which means the smelters cover a smaller area, which means more smelters and more expense, or they're investing in that monitoring you're talking about, which isn't really useful unless something's close enough to react to what's detected or someone does a nab-and-jump, so more boats. Post boats along the route, or send boats along with the ore as escort - frankly it'd cost about the same to load it onto something that can get it there under thrust, and it'd get there faster so you could use the boat for other shipments - you're spending maybe Cr60,000 or more per boat per month for something cheap.
Whether it's more boats or more smelters, overhead builds up quick, cuts into profits pretty deep. Areas with rich enough concentrations might be worth it, but other areas aren't going to be worth that kind of vigilance. Either the ore is flying unguarded or the mining crews are loading it up and flying it back themselves 'cause it's cheaper to fly for a few hours and then get back to work than to cut their profit margin with all that extra overhead. So, a lot of places aren't going to be using the ore-pushing idea because the expense of keeping it secure isn't going to be justified by the mineral value being pulled out of that chunk of belt real estate.
Second, putting aside the relative uselessness of the actual x-boat network for important communications, that image of sending the message out and everybody jumping to enforce the law is something that might happen in Core sector, but out here in the Marches there is just too much frontier for that to work. Lotsa D and E ports, lotsa struggling low pop worlds without the resources to respond to such a call - and a lot of those more likely to take the inbound criminal's money, strip the cargo of any identifying elements, and then process it down and blend it in with their own output before anybody can show up to ask questions. About 30% of Marches worlds are D/E ports.
And then there's all that open space: entire star systems with nothing more than a few thousand or tens of thousands of people on one world and absolutely no one in the remaining quintillions of square kilometers, where an organized crime syndicate could stick a mobile processing operation in some distant corner and not be found if half the Imperial Navy showed up to look for you. Roughly a third of the Marches systems are under 100,000 population, about a fifth under 10,000 population.
The frontier is frontier.
But, let's look at that communication network. You're either getting the news out to the x-boat network, and then waiting for news to travel out from the various network links, which by that time the ore's been processed down someplace and is gone, or you're building an effective network that gets news out to all the places the thief could go within a day of the thief getting there, which means enough ships in your system to reach every system within about jump 3 - and then some extra because the easy crime syndicate solution to that is to stage a theft, have the first ship abandon the cargo in some other system and flee immediately, and plan a second theft after all those ships launched to spread word. Again, starts getting expensive unless the Imperial government's doing it at their own expense. Of course, at that point the Imperium might be spending more than the system's actually producing.
You could set up a civilized sector that could do all those things, but it wouldn't look much like the Marches. Wouldn't look like a lot of the frontier sectors that players tend to play in.