I don't like this idea, for the same reasons that other have already given. Grafting histories with git-replace might be viable, but it'd still be clunky and non-intuitive.
Why don't we just suggest that people use shallow clones? Git supports pushing from and pulling to them since 1.9, and while Gerrit doesn't accept pushes from them (or at least it didn't when I just tried), I see no reason why Phabricator would have any issues if it only works on diffs anyway, not commits.