It depends. "Gassiness" is the kind of thing that would take a bit of preparation. Some MAs are open to more simpler fetishes (like feet or prostate play) right away, but I think your case might be a bit more different. It wouldn't hurt to ask, but I imagine most would probably say no (either because they're apprehensive to try it, or they simply can't fart on command).
I recommend finding an MA you can enjoy vanilla sessions with, then open up to them about your request after a few sessions; that way, you can build some chemistry together, and they have some time to prepare before your next session.