I'm going to echo Martin's sentiments.
I have a friend who used to run an online shirt store and usually did a limited run of two or three screenprinted t-shirt designs; she'd cycle into new designs every three or four months, and by the end of the year she had a small back catalogue people could order from, and whatever was sold out was gone unless she did a second run.
If everyone settles on a design I could try to talk her into getting all the equipment out again or even borrow it. Barring that, screenprinting supplies are fairly cheap, with the only problematic (expensive) supply being shirts. Her prices were around 10 dollars/tee (and that's WITH a profit), so assuming enough people are interested, I'd sell them at-cost.
Also this gives me a good excuse to get into screenprinting (I've always wanted one).
For really basic designs (like the MAGMA shirt) that can easily be done with a stencil and some fabric paint.
I rather go for posters than T-shirts.
I can do posters too with screenprinting.