I don't know that I've ever wanted to diff a diff, but you could do that still. And bisecting would still be possible, especially if you use merges instead of rebases.
Bisect rebases... you mean that you have two release branches based on divergent upstream branches and you want to quickly test where a bug was introduced on the way from the one to the other? What I would do in a rebase workflow is find the merge base (`git merge-base`) of the two release branches, and bisect from that to the release branch I'm interested in.