Finally got around to trying this. The screenshots are hard to appreciate, but trying it out I have to say it is very fun and refreshing! I have a couple questions:
Won't using any brush settings that affect transparency introduce the RGB blending effects that this is trying to avoid? So to get the best "real pigment" blending we should disable radius-by-random, pixel feather, hardness, eraser, opacity, and any other settings that might create transparent dabs?
My other question is probably (very likely) naive, but I was thinking about the saturation problem you mentioned. If you convert to RYB space couldn't you do a straight-line to determine the proper chroma and hue of the dab? Ignoring the tinting strengths of real pigments, of course, unless we want to reference a table of every hue and its strength as part of the formula? Yellow is weak, Blue is strong, etc.
Handprint talks about this problem quite a bit.
There's some info on converting to RYB and back to RGB here:
If using RYB wouldn't that solve all the "wrong way" CW vs CCW issues? Red and Cyan and Blue/Yellow are not anywhere near 180 on the RYB wheel, more like 120. Likewise, if we use RYB the 180 degree complements should all mix toward a grey-- that's what we want if we are talking about pigment/real mixing, right? Red+Green == Grey. Purple+Yellow == Grey, etc?. Of course if your two opposite colors are of different Chroma then the result should lean towards one or the other.
Then again, maybe we don't like grey at all. I do find it fun that if painting an intense yellow over a dull blue I will get intense greens popping out here and there unexpectedly. It's not necessarily "realistic" but maybe that's not the goal after all. . .