I would never wave my willy at a Yes gig!
That said, my own willy-waving claim to t-shirt fame is owning one of the members of Pale Saints own shirt from the early days. Complete coincidence really. I had a search on eBay for one of their shirts, and the first one up happened to being sold by their ex-guitarist, 'as worn at various of their gigs'. It cost me what it would have cost new, so top deal all round. I wear it quite a bit. They obviously made them to last (it's from c. 1988/89).