You guys don't actually need to make them buyable on a base you know... (Talking to the admins here)
If you leave the buy code out on the base in market_ships.ini, the ship still exists ingame. So, you can use your admin ping powers and just change the persons ship etc etc.
Just saying, so we don't need to add base after base...
Actually, there's also the Kusari imperial yacht (not the regular yacht, this one's the Emperor's flagship), and the Barge, although that's more hugely rare than unique as such.
If the player can keep up a high level of quality RP, stick to fairplay ethics, and be active so a wide range of players can interact with them: I don't have a problem with individuals getting a custom ship.