This might sound bad but...

I cannot actually see a problem with #2, it would be my preferred solution. When I go to trade shows / conferences it is to talk to brains, not bodies, the sex of the brain is immaterial to me.

To me, making sure the bodies are 50% equal in gender means nothing if 80% of the female bodies are only there to make up a body quota but have no interest / knowledge of what is going on.

Maybe change it slightly so that the rules state anyone at the booth must be a full-time employee or a contract employee with X months at the company.

Could you go into a bit more detail about your reasoning for #2 not being suitable?

[EDIT] Messed up my percentages