Its a tough one as only you know your partner. IMHO, open and honest is good and the correct way, but be prepared that she may not take it well. Accidental porn is often spoken of, however, my only concern is how 'accidental' can Zoo porn be? Its not really main stream stuff. She may well get the impression that the accidental bit was contrived and that would not go down well.
I would try an open and honest conversation about sexual fantasies, in the right setting, edging towards Zoo, but leaving you able to back out without committing too much of yourself if things don't go well. You can amp up the conversation or back off as you gauge her reaction to what you say. If all goes well you can share more and ultimately admit your feelings. This may not be a one night thing, start slow and see what happens. That said, I used the open and honest disclosure but, like
@Bucephalus , I knew and trusted my partner and was confident of a positive outcome, which we got.
Only you know the answer. Prepare for the worst and hope for the best.