Possible to drain mini split into clean out opening?
Are you against installing a sump basin in the cellar and having the split drain into that, with a sump pump to push the water outside of the house? Maybe this is financially prohibitive, but I suspect you could DIY it relatively inexpensively.

That said, I'm not a plumber, but you could optionally add a 45-degree branch, shaped like the pic below, extending up from the "house-side" cleanout access. In your case, I would keep the vertical run capped, basically just extending the existing clean out upward a couple feet, and drain the mini-split into the 45-degree branch.

.......there are some caveats, for which a plumber's experience would be desirable: 

1) What happens if/when you have a backup? Where does that problem manifest itself?? It should backup into the next lowest point, which is probably a bathtub, but the mini-split will be introduced as an option!

2) Venting. The dripping from the split probably doesn't need a vent to flow, but you might acquire a sewer smell in that bedroom. This might even be illegal in some respects, like if the home or room is rented.

3) You might hear gurgling sounds in that room, as water flowing through the piping will be transmitted up to the evap/fan assembly. Washing machines draining, toilets flushing, and bathtubs draining might be enough flow to transmit the gurgling sounds.

I would advise the sump basin/pump strategy, but the above could probably suffice as a short-term solution in the meantime. Again, I'm not a plumber!