I think it's a good idea. Not everyone will wear any type of t-shirt, so maybe you should find a way to match people to brands. For example, I'd wear a t-shirt advertising technology companies whose products I use, but I wouldn't wear a Ford t-shirt because I am a Toyota/Honda guy.

Someone launched a similar site years ago where they would give you giant stickers to put on your car doors and pay you $200 per month as long as you drive 20+ miles per day.