My instructor was always keen to avoid an inverted spin, even stall turns were done with 80 degree upslope. Aparntly the robin can go inverted easy.
I've been taught that you can't spin until you stall. And an inverted stall in the R2160 (assuming you're trimmed for a fast cruise, which is what we use for aerobatics) is hard. In fact, with the seat at its rearmost setting my arms are not long enough to get it to really do an inverted stall.
In any case, you are nowhere near stalling, upright or inverted, in a stall turn. I'm very happy doing them with a 90-degree upline. What you do need to watch out for, though, is to exceed the 90-degree upline as that will stall the engine. That'll make the turn very sloppy and lead to a significant net altitude loss.