I wonder if there would be some way to do this as a "print on demand" sort of thing. Correct me if I'm wrong because I've never ordered one, but don't they have something like that for T-Shirts?

There looks to be plenty around. Most are 'contact us for quote'.

UK and NZ: http://www.mankymaps.com/

Australia: http://www.digitalfabrics.com.au/

Australia (Wallpaper to any size!): http://www.pickawall.com.au/

Australia (any material including leather!!!): http://www.visual-imprint.com.au/