Seems like a hard problem to solve. Sending people to do product promotional is perfectly legitimate depending on the type of event. Even if you weren't deliberately targeting a male audience with attractive females (although I'm sure that was the case), odds are that if you were recruiting for that kind of work 90% would be outgoing and attractive young women. Just like recruiting programmers will get you 90% geeky…
The problematic assumption here is that a given woman was paid to be there. This was a reasonable assumption on the part of the other attendees, only because there are so few women in the games industry.
So rather than the problem-to-tackle being a cat-and-mouse with vendors trying to get the most-effective promotions' staff to the venue, the problem-to-tackle is women in gaming.
If women could hit even a respectably-sized minority of the attendee audience, the assumption would no longer be safe.