After quite a lot of trial and error on the feeder, I've settled on a design where one end is a hinge, forcing the vibrating motion to be confined to up-down only, and with low amplitude at the hinge end, and a large amplitude at the free end. It seems to work well at separating parts as long as there aren't too many at once:

Obvs this is a dirty quick prototype using scraps lying around, I'll do a neat version one day out of profile aluminium...