If it was just one thing you wanted to know it could possibly be put on the membership card thing and explained that it was for a program item. That we were trying to figure out how diverse the convention is in one particular way and have a discussion about it at such and such a time in salon B. There might be a few people who didn't care to fill it out, but if it was just one thing most people would go along. But if it went beyond one measure then something else would have to be done. Perhaps a station near registration with the incentive to sit down at a computer and answer questions to get entered in a raffle. Thought there would be a bit of a problem having enough robust computers to do the task, one wouldn't cut it. Though perhaps that could be something to do with all those older computers we all have sitting around doing nothing. Load some variety of linux and some sort of tabulation survey program thing.
It would still take work though and for one program item I don't know how many conventions would want to go to the trouble.