Woah! That looks amazing! Definitely something I would print out and laminate! If you could seriously do this, then I would really owe you one, big time!
I know it is only a prototype, but if possible, I would make the card larger and I'll explain why. I'm not too sure how familiar you are with Skylanders, but there are some figures called the Sidekicks which never came with cards like every other figure did. Darkspyro of Darkspyro forums (duh lol) went ahead and created custom cards for each figure. Here is a link for a reference.
http://www.darkspyro.net/images/skylanders/custom_sidekicks_small.jpgAs you can see, the cards are quite large. I am not sure how other members of that forum did it, but the way I printed it was like this. First I scanned into my computer the back of one of my Skylander cards. I did this for two reasons; to get the size of the card correct and so that I can place an image of the back next to the image of the front for each card (to be folded over once printed). Because the image of each card was so large, it made it easy to resize each one without having them look blurry or distorted. So the point of all this is that It is always easier to shink something than to enlarge it, but I'm sure you already knew that.
And honestly, instead of a random code for the back of the card, maybe you can make custom ones for members who request it with their own code? All of that through PM of course, so that the code isn't public knowledge. I know I would prefer the card to have my code than just a random one.
Thanks again for doing this, Toops. You're the best!