I also wonder if there are any ethically motivated vegetarians who refuse to eat animals but don't have a philosophical objection to eating human flesh (perhaps considering it a symmetric kind of justice).

I have no ethical qualms about eating humans, no. Assuming it is freely given, of course (animal flesh fails ethically on that point; interspecies communication is simply not good enough to convey consent).

Other classes of objection do apply, though - having been a vegetarian for seven years or so, could my digestive system handle flesh without being upset? What about pathogens - they're bound to migrate more readily when predator and prey are the same species; will it be worth the risk? I think not.