This is great, I love creative challenges like that. I too would like to find a way to create t Shirts for a niche audience. It's one of those things that will have to wait for now but I'll always be thinking of how I could use it in future.
I like coming up with marketing copy, quotes, idioms, lyrical stuff, poetic devices.
I remember at college I made a couple of t shirts using iron transfer paper as an experiment. I would most likely use some kind of QR code as part of the design.