There's lots of wacky ways to market - bumper stickers, fences, balloons - anything you can print on really. If it worked, that's good. I don't know if it would work in all markets on all listings though! The real key is where you would be wearing the shirt!
Yeah, this is a new one to me. I've seen shirts promoting the business, decalled moving vans, etc. I don't really see where a listing on a shirt would help much.