Hmm that is interesting. I have often wished for a mask by subtool. I have fiddled with a few different ways. The closest I have come is by using duplicating, Merging the two subtools together, then mask by AO with a really tight setting. Then manually filling in the inside. Still pretty time consuming.
Another idea I had was to make a union Boolean, fill it with polypaint, then project the polypaint, then mask by hue. That didn’t work out so well. Might take some more fiddling.
The stencil thing isn’t bad either… it’s just a shame you can’t seem to convert that to a mask.
We can cross our fingers for the next update I guess!