I think it can be accomplished with muscular action. There's not a lot of vaginal room and I believe the process is a more gentle version of solid waste removal (and for that matter urine removal as well - we don't pee because of gravity). Also a little regular cleaning during the time would be of help.
Mousthief is correct about peeing into a vacuum. That's pretty much how they do it. When you think about it, it may be easy for a man to pee in space but where does it all go without some other encouragement?
I'm going to ask NASA as well - maybe this should be a concerted effort. You know how government is when it comes to answering questions they don't want th answer.