Nice! For some reason I had thought you were here in California, but I see that you're not.
I'd say throwing a simple design together isn't that hard (I've done some things like it) and I know some artists who could advise. But, I think to print shirts or large numbers of stickers, we'd have to collect money in advance, which means a level of logistical hassle and bookkeeping that I don't think we've really considered so far. If we want just a few stickers (like 1 or 2 per person attending the conference) and don't mind them looking kludgy, we could make them on a laser or ink jet printer. There is also the issue that people who don't attend the conference in person might still want t-shirts etc.
Simplest might be to just upload a design to someplace like Zazzle or Cafe Press and let people order their own shirts (those places can also make stuff like coffee mugs from the design). At that point I wonder what anyone gets out of it, though.