I had a thought, and just got done testing it. Heh.
I opened WB, opened castle_gondorne.bse, and added a plot in front of the citadel. I saved it.
I DID NOT HEX EDIT IT. The file header still contained castle_gondorne, all lowercase. This is important.
Next I opened civilianbuildings.ini. I edited CastleFlagNE such that the unpack line was like:
CastleToUnpackForFaction = Gondor castlegondor_ne ... blahblah
Note that I used an all lowercase string in the unpack line.
Yes, it worked in game.
This obviates hex editing the BSE files for new factions, and a simple case sensitive replace all would work for edited existing BSE files.
Take care.