Github has a really great url api - you can usually do git related compares right there in the url - github.com/some/repo/compare/HEAD...comitsha is actually a thing.
And while not particularly ergonomic you can have PR against other PR since forever.
Github is built by devs for devs and has tons of hidden gems scattered in there. Do read their shortcuts help, blog and other dev sources and you might find tools for a lot of the task you can think of.
That feature breaks down in the presence of merges. But when those are not present it works OK, you see lines like this:
> force-pushed the aojea:affinity branch from 8eab3f7 to ee626e7 yesterday [0]
Where the "force pushed" links to a pretty decent UI [1], but it will include a bunch of non-import information if the person rebased and force pushed, which is a common reason for force pushing.
You can see that in this [2] long lived Kubernetes PR that was rebased, the resulting "helpful" link shows me 2094 changed files [3]. In a tool like Gerrit or Phabricator this is automatically handled and it would basically ignore this operation unless the rebase changed the code being reviewed.
And while not particularly ergonomic you can have PR against other PR since forever.
Github is built by devs for devs and has tons of hidden gems scattered in there. Do read their shortcuts help, blog and other dev sources and you might find tools for a lot of the task you can think of.