I had written a very long reply, but a damn database error send it to Oblivion!
Tom, it's even worse than that.
If you try to add more than 256 buildings, let say 266. When you create a city, a building will be added automatically (building #258 for instance). So you make thing "not a problem. Since 258 is added, I will copy it to 267, and make 258 useless. so it will have no effect in game, but the new 267 will compensate".
Except when you start your game, now that you have 11 extra buildings instead of 10, you may start new cities with building 259 and 263 instead of 258...
In other words, the extra buildings are not the same depending on how many buildings you have added, and I couldn't find a pattern for it.
Another fine example of Firaxis exotic programming.
This being said, I have plans to make a new version of my editor that will allow more than 256 buildings, and more than 32 resources, and more than 4 eras...
To allow this, each buildings, units, resources, etc will have a "starting era" and an "ending era". And you will still be limited to 256 buildings available at the same time for 4 consecutive eras.