In your position I don't imagine I'd want or need to have any contact with the potential metamour either. Life is too short to make nice with people you don't like, if you won't be spending time together.
Unless I had so much in common with a long distance metamour that I'd really hit it off as friends with them - well we know long distance relationships are hard enough to maintain, no reason to start new ones. If you aren't looking for pen pals, why would you want to invest energy in a relationship like that?
I might tell him that she could contact me if she wanted (if you are OK with that, which it seems like you aren't), but I'd certainly say I don't have any intention of contacting her, and I'd tell him why I didn't desire a friendship with her. Not sure why he is putting the onus on you. What you are suggesting isn't anywhere near a DADT, and there's no reason to feel badly. Wanting metamours to be friends and insisting on it are two different things, and I hope he doesn't try to make you out to be a bad person just because you don't feel like making yourself feel uncomfortable to make him happy.
Happiness will never come to those who fail to appreciate what they already have.