That's an excellent idea. You could have a two-letter code Y*, where the second alphanumeric was the maximum hull size (So Y1 mean a yard that makes 100T ships only, YS means up to 200,000T hulls, etc.) Type of vessels built are always both ships and boats, except when TL is pre-stellar. Total tonnage tend towards !T per 1000 population, but may vary by as much as one order of magnitude.That (splitting yard from port) seems a simple elegant solution. Especially as there's already a disconnect between the two with planetary govs being able to build without a Class A or B Starport rating. And making it an extra code bit would work well enough I think, it could even be easily classed by hull size capability then, rather than non-starship or starship (as I see no reason to differentiate on that either, let it be TL and hull size that determines that).
You'd have to come up with some other way to distinguish between Class A and Class B, but other than that I see no objections at all.
Modifiers would include population size and volume of through traffic (what trade routes is the world on).I wonder too if maybe the Starport TL mod wouldn't be more appropriately a Shipyard TL mod? Not that talking such ideas out on the forum is going to influence anything official of course...
Hans