I had a vintage German bike where the leather front mudflap was attached via two hooks around the sides of the fender. It could slide up the fender, then would fall back down until it caught on the fender stay. So if you rolled the bike off a curb, rather than smashing your flap it would just slide up, then fall back down. Simple and brilliant.

Edit: I recall it was a Puch. Too small for me, I gave it to my neice.