Ahriman
Tyrant
The "American Dream" mod, for example, does its thing by silently making a UB out of the Palace for that civ with the "trait" abilities, but it makes things awkward by having two entries for "Palace" show up in the Civilopedia.
The only way something like that wouldn't be awkward would be to embrace it and have every civ have a free "Unique Palace" with America's White House, Germany's Reichstag, etc.
That's a good idea, but I imagine there are still really limits to what you can do with buildings.