I wonder if the host hotel might be willing to print up a special room key for convention attendees? I don't know how much the cost would be but it might be presented as a way to increase the number of convention attendees staying at the host hotel? Might want to suggest that it be announced that only two of the "commemorative" keys would be given out with each room rental and if anyone "lost" their key they'd they'd be given a regular room key as a replacement.