I've been experimenting with adding buffs to custom CAS parts, and it doesn't seem to be working. I noticed there was a field in the CASP file called "BuffResKey" which points to a buff file, and "TGI...
UPDATE: OK, so it seems that it WILL work, but only with a GroupID of 0x00000000. Clothing Buffs on CC with a GroupID of 0x80000000 will NOT work in-game.