Well, duh, as soon as you throw an idea out here, the "cover" is blown.
I don't think you mean the same as me when I say "your cover is blown". I'm referring to the fact that whatever "it" is, it has to not identify the indivdual. People like me who are still "in" would not do something like putting a mark on a badge. I would, however, leave a copy of a bible with "JWD" on the inside cover.
The point of the thing would be two-fold. Firstly it would make people like me aware of others present in the same situation. Secondly, I would think at every convention there would be someone privvy to reports of "x number of keys being found" etc that could be reported back here. I would certainly be in a position to find out for our convention.