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…
I agree that it's a hard problem, but I think you're looking at the wrong one. 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,…
That's what I find so strange about the original post. She's assuming they're there as eye candy and they sure don't seem to be. It's odd to hear a woman assume that other women (especially since they're pretty average) are there for their looks. The few I've talked to were far too knowledgeable to be some sort of incognito booth babes.
Booth babes are usually given a few talking points and know little to nothing else about the product beyond that. They pass you off to the nerd the second you appear interested.