In no particular order....
Get her drunk then bring it up. If it goes well, fine, if not, blame the alcohol and plead no contest.
Find a forum where the *mystery* act is discussed and ask them how they talked their SO into it.
Rent a movie featuring this act (not a porn flick but an otherwise ok flik that has a scene of it). Say something interesting about it when it happens to lower her guard about it then check her reaction. May take some homework to find a movie with someone's Mom and a horse (
)but I guarantee there's one out there somewhere.
One other suggestion is to just ask her how she feels about it. If you can't be open about what you really want, that's not entirely good. Sounds like you are afraid she will judge you for it - is that who you want to place your trust in? I'm not asking but you might want to.
Lastly, you need to decide how important *it* is. If you can't be happy with someone who doesn't enjoy *the act* then you might as well come out with it - if she likes it, you are in business if not, time to cut your losses.