I'll agree with the member names idea, although I know it can be a bit of a pain.
We did this a few years back on another forum I'm a member of (for a now defunct band that was popular in the midwest), and people still recgonize me by my screen name when I'm out and about and wearing my long sleeve t-shirt. On the licence covers would be doubly awesome as it'd be fun to recgonize your fellow freaks by username.
I know that we used cafepress as well and I think the site owner had to go in and actually create the shirt for each of us that wanted our usernames on a shirt using their software. I'm not sure of the specifics as I've never used cafepress myself, but I can ask him if enough people are interested.
