Spoilers for Book Six.
Do not read further if you don't want spoilers for Nightfall. Some of the comments here also contain spoilers for earlier books. Further down, I have a comment with spoilers for Flashback as well.
So, in book six, we learn that Mr. Forkle can no longer be in two places at once. This means that if he (or one of his known identities) is standing in the same room with another character... he can't be that character.
This may not actually be worth much... but it will help us start narrowing down the list of people that he might be.
So, the following is a list of people that Mr. Forkle has appeared with during book six. Some of these will be very obvious (like, duh, he isn't Sophie; he's probably not a goblin, either), but I'm just trying to cover all the bases here.
Mr. Forkle CANNOT be:
Lady Gisela (didn't appear together physically, but she was hailed while Mr. Forkle was in the next room over from Sophie and Keefe)