Slightly OT- I once saw a jelly jar that had been removed from a man in the OR. Wanna guess where they removed it from?
Name of the site escapes me, but there's a video of a dude shoving a jelly jar in his ass, then sitting on it, causing it to break, then pulling glass shards out of his bleeding ass. Pretty rough ridin' disgusting, so I wouldn't suggest shoving a jar up your ass in an attempt to catch a fart.
I think you could catch a fart in the tub, though you'd have some water in the jar with it. Not sure if that would make the fart-in-a-jar better or worse, but seems like an interesting thing to try.