I heard of a company that had QR codes on the t-shirts and paid per scan. (also, if its direct response advertising you can use unique phone numbers, unique web addresses, and other typical tracking methods.) There is an angle here if you understand the local printer business. The company I heard about was going to local jersey printing companies and sponsoring/subsidizing uniform and jersey printing for local high school sports teams and was working with brands like RightGuard. By tapping into networks of printers, they could bring major advertisers on board with real scale.