Most characters who are not the Champion do not have a full stat block and all of their descriptive text is written by hand rather than being a whole mass of parser calls. That means that any scene using transformative accessories would also have to be written by hand, for every variation you might want to do. Needless to say, this is not a small undertaking. There's a reason that Companion TFing is limited to a set number of variables that their writers can plan around. Once you start trying to apply that to a broader selection of characters the amount of text would skyrocket.I Think I already brought this up in some small capacity in another post on this thread, but I really would like to see all of Viviane's more unique items being usable on specific characters. Enemies, allies, neutral parties, it's all the same to me. It just feels like the massive potential they have is limited by the fact that only the pc can use them.
Now, I think that if someone wanted to write a new NPC focused around the use of these items it would probably be accepted if it's done well enough and I could see maybe a specific scene or two written for existing characters and a particular item (assuming their writers are okay with it) but I wouldn't hold my breath waiting for anyone else to do that.