Update for completeness: I did implement a 4-level "port_buildings tech tree" with "docks" being the lowest level at large_town (trade fleet only, no warships). Then I copied and renamed the "barbarian port TGA-s" in the appropriate faction \UI\faction\BUILDINGS and \UI\faction\BUILDINGS\CONSTRUCTION directories and added the appropriate text descriptions. RTW accepted my changes. From testing, the AI seemed perfectly happy with building "docks" prior to "ports". Large towns got the appropriate trade income. The AI ship levels looked reasonable and the AI was still generating more high-level units by midgame. Looks like a success to me (at least until the forthcoming CA Patch resets all of our tricks).