I think that sounds like a good idea - I have been thinking about building a card that pulls together all the sites that I manage (been also thinking about a website for them all too, but that's another story).
A while back I wrote a post on designing social media business cards. When you go to trade shows, events, etc. you still need something to hand out to people and I think the idea of adding your social media handles (or even a QR code like you suggested) would be a useful business card.
Here's my post on social media business cards: