The one piece of circumstantial evidence I've seen discussed
ad nauseam in the other thread is the fact that one of the spectators who was "hypnotized" in one of Derren's shows turned out to be an actress who afterwards listed that appearance on her IMDB profile for a while.
I think that is far from conclusive regarding Derren's use of stooges, as the word is usually defined.
I recently watched one of the talks from a past
Essential Magic Conference (an "online convention" for magicians) discussing a TV pilot shot a few years back for UK TV. This was a mentalism show which utilized some fake hypnotism, and the speaker said that no stooges had been used. He did say, however, that the audience had been carefully selected, and that they had invited members of the local acting group, because it was thought they would be able to provide the desired responses.
We know Derren carefully selects his audience members, and it's not a stretch to think that he would also invite actors, who might be more able to deliver the desired "peformance". If an aspiring actor is lucky enough to get selected and make it into a famous performer's show, it wouldn't be unrealistic to expect him or her to list this in their CV, even if they hadn't actually been hired to perform. The fact that she later dropped the reference may mean she wasn't actually hired at all.
Whether you consider such a person to be a "stooge" or just someone who is good at pretending (and chosen for that reason) depends on your definition.
I just think it's a part of a professional magic and mentalism performance. Nothing to get too excited about.