suspect it would also diminish most of the social and psychological benefits of vegetarianism -- as willpower training, proof to yourself of your own virtue, proof to others of your virtue, etc. Still, this might be a good option for EAists to consider.

I can train my willpower on many other situations, so that's not an issue. So it's about the virtue, or more precisely, signalling. Well, depending on one's mindset, one can find a "feeling of virtue" even in this. Whether the partial vegetarianism is easier to spread than full vegetarianism, I don't know -- and that is probably the most important part. But some people spreading full vegetarianism, and other people spreading partial vegetarianism where the former fail, feels like a good solution.