Tough one.
If you're going through the cards and find one without a signature its a DQ. Surely if you also can't find a card it has to be the same thing...?
There's also the problem that if it gets out that you're ignoring some rules for certain people you'll get the sticklers going ape, and also the next person that screws up will demand a concession.
As painful as it is, and a little unfair, DQ.
This...
Ive been DQ twice for forgetting to sign the card, still got cut both times