The branch is a wad of metadata included in (attached to?) the commit. It'll somehow have to roundtrip the various metadata. I doubt it'll ever be 100% complet

The branch is a wad of metadata included in (attached to?) the commit. It'll somehow have to roundtrip the various metadata. I doubt it'll ever be 100% complete, but you should be able to get quite a ways. Certainly far enough to make it work using github as your repo of record (and as someone who regularly submits github PRs via hg-git, I have evidence that it can work.)