I'd hope that by that point I'd have sold them on my company enough that they really thought it was better.
Also, I don't think it needs to be a long gig. Maybe even just a day, which isn't much longer than the gauntlet of technical interviews some companies will put you through. Even that short period should be enough to see if someone works well.
Doing a day's contracting elsewhere would violate my current employment contract. (And I think strict adherence to the letter of the rules will be reasonably common among good developers)
Also, I don't think it needs to be a long gig. Maybe even just a day, which isn't much longer than the gauntlet of technical interviews some companies will put you through. Even that short period should be enough to see if someone works well.