Honestly, depending on the con's demographic of attendees and the display options, I might just suggest the con buy the print(s) outright, or pay the artist to get a cheaper copy made. If you get a bad group, the print could end up stolen or vandalized, and then the artist is out the cost of the print. Heck, the attendees might not even do it maliciously! I remember a number of years ago that some of AI's signs were ...co-opted by some attendees at night, and had directions to a room party written on the back. But in general, this being a great opportunity for artists to advertise is right on the money.