I think that the tile availability choices in the customization mode are decided by the client based on the era flag provided by the server. I see nothing in the housing code that mentions era.
I can solve the mystery of the older posts above here though: Newer clients have a different offset for the tile values. Once that is corrected the right pieces show up where expected. It has been so long I forget which script that value is located in. A few years ago I updated our RunUO RC1 server enough to move from client 5.0.9.c to 7.0.x, totally in place with no shard wipe. Now that was an endeavor! There's none of the "real" SA content but the land mass is there, gargoyles can fly on the paths, etc.