The PR into their branch sounds nice, but not always necessary. If you forked the PR branch, threw a commit on top, opened and accepted a PR from your-branch to main and declined theirs, would that work? They get their commit in the history, you make the changes you want without back-and-forth. You could link from their-declined PR to your-accepted PR.

I guess that's messy if it's linked to issues and the connotation of "declined PR" isn't as nice.