You're going to have to make a separate repo for everything anyways, because you'll have to change all the init scripts and whatnot that get supplied with programs... well, unless you install both and just leave the useless systemd files laying around forever.
My /usr/lib/systemd takes 6.4 megabytes, so I think I could live with that overhead. Or I suppose just delete them manually.
What would actually be useful would be a generic OpenRC wrapper that would ingest those service files and provide traditional start/stop interface for them.