I'm looking through the areas of data.pack related to buildings... and I don't quite know where to start. I figure there are two ways. One is to create versions of eastern/desert buildings for the Mauretanian factions (just Mauri and Gaetulians I think) which are tagged to have Roman graphics. This sounds easier to me, but given that Eastern and Roman building trees don't line up perfectly, it may present some difficulties. The other option I see is to create a version of Roman buildings, assign desert unit unlocks to them, and give them to the Mauri and Gaetulians.

Figuring out the specific steps involved in either path will involve a lot of digging and trial and error, and I hope I can glean some knowledge from people more experienced than me at modding faction buildings.