I will
definitely sign my cards! I've already signed a bunch for the Mage Wars staff. I'll be working in the demo area the whole show, so bring them by!
I'm not sure about the question of using signed cards.
For this GenCon, my guess is that we will be handing out spellbooks for people to use, since the game is not out yet. This is how we've been doing the tournaments up to this point. So, you would most likely have to use the demo books.
In future tournaments, I don't think it would be a problem, but I'll ask around.