A better idea for adding a sig to a contact print, if you have just one sig to apply to many prints:
Use film! Take a sheet that's unexposed, process it to get rid of the emulsion, and you'll have a clear sheet of perfectly-sized plastic film to write your signature on. And a sheet of film would line up perfectly with other sheets of film.
You'd probably only want one sig for an entire event, as it would be a rather expensive use of film if everybody got their own signature.... Even if you were doing it on (cheaper) mylar, I'm not sure I'd want to have to make a bunch of different sigs for an event.
I'm going to try it, maybe tonight. I've got some 4x5 contact prints to do... just need to figure out where to get ink that is opaque. I don't know if permanent black marker would be enough.