Unless you're going to bring along your own enema kit, I think you'll be stuck taking your chances. I've screwed a few WGs in the ass, and they've never pooped as a precursor to anal. I haven't had a problem with anything coming out of any WGs ass, but as DD said it's always a risk.
If you really want dirty, try talking a WG into a2p, or a2p2a2p2a... etc. For the uninitiated, that going straight from her ass to pussy (or back and forth) without washing or changing condoms.That's really dirty, and likely to lead to the smelly pussy infections that Asslover was complaining about here (
http://forum.sex141.com/eforum/viewthread.php?tid=10505).
-G