In europe there are a lot of touring bikes built for a smaller wheel. I think it might be a 650b and they also are fitted to mixtes but I have only seen one used one for sale here here.

Depending on the frame you are using the gap may not be that big. Modern racing frames have very little gap so the smaller wheel and fenders could work and look fine. Mount the fenders for a 559 on an extender you'll have to fabricate from the fender to the brake bridge. As long as brakes aren't an issue I think you are good! Ron