What about surveillance cameras at the hotel? It's a long shot but maybe it was caught on video. I'd contact security to see if it was.
However, I'm still curious about the weekend pass and the database of attendees. If it exists, then the promoter might release the list to you provided all the really private data is redacted. It would be kinda like a newspaper publishing the names of people that attended a local marathon or something, so it's not a big deal or request. Or better yet, the promoter could post the list of names on their website or Facebook page thanking all that attended for their support. Perfectly legit. You could always tell them that it would be in their best interest to help you out because they don't want people like this at their events. So it's a win win.
Anyway, with the list you could obviously eliminate the female names. By process of elimination you could narrow the search of the male attendees down to the ones that are closest to the names you suspect or that are about the same length as depicted on the name badge. With the list of suspects, you can then Google the names and/or search for them on Facebook. You might get lucky and find a match that you can contact.
Could you bring criminal charges against them? Most likely, no. Again, it can't be proven that he actually removed it from the premises, and quite frankly the police have more important matters to deal with. But you can always appeal to him for its return, and if he won't then the public shame and humiliation of outing the d-bag could make it all worthwhile, and just think of the lulz if you found him.