It already is? If I visit a single commit's page, in the Options drop-down on the top right-hand side I can click Revert, and a modal pops up asking me in which branch I want to revert that commit.

Yes there is that dropdown. But when you select your feature branch and click submit, some magic happens that does not do what you think it does.

Hmm, what magic is that? It appears to create a new branch with the revert, and create a merge request from that branch to the branch I select - which is what I expected. This is of course with "Start a new merge request with these changes" checked. When it's unchecked, I'd expect it to make the revert commit directly on that branch; is that not what happens?

I just checked, user Vinnl is correct that you can already do this for every commit, and targeting every branch.