OpenRC predates systemd I believe
One of the pmOS devs wrote a user session service manager [1] but pmOS doesn't use it (yet?). And Alpine as a whole is planning on moving away from OpenRC anyway.
i am working on a implementation of that for openrc for a while now, it's been working nicely so far: https://github.com/OpenRC/openrc/pull/573
I'm not totally sure whether openrc will merge this or not, but I would definitely like to see this be added.
edit: They actually seem to have replied to you while I was writing the original comment.