I picked up one of these a long time back knowing it would save my day, make a friend or both:
Wheels Manufacturing in Boulder makes all kinds of cool bike bits.
Bogart had an even better idea with the addition of a chainring bolt convert your vinty mixte into a solid hanger style drop:
There were an awfully lot of Peugeot, Raliegh, Gitane and Libetas mixtes made with dropouts without hangers. My theory: the frames could be sold as 3 or 10 speed bikes. What's yours?