As luck would have it the talented Michael Pavlovichcovered this in his last tutorial!
A: backface masking + setting “Depth”
Q: I Know ZB can do it but forgot how to set the Mask brush (also can work with polypaint) to constrain to hard edges, or at least when it meets a 90 degree edge.
There’s a tutorial somewhere on the Internets about it. please post link or setting here, thanks!