...And as it's something that won't die, I shall bring it up again.
The Throne Room may become a reality after all... It's already "active" in Civ 4! (Sort of.)
I was messing around and discovered this accidentally... don't know if someone has already found it, but you can actually preview the Throne Room in BtS 3.17. Start a Civ 4 game and press Ctrl + D in-game. This will bring up the debug screen. You will see an option marked "Throne Room." Check it, and watch what happens!
Surely a modder acquainted with the interface code can find out how to expand this by adding the ability to upgrade different portions?
EDIT: You may have to have "CheatCode" set to "chipotle" for this to work.