It's the reason why most of the times I'd rather paint in Zbrush.
Because painting in ZBrush is fully non-destructive, right?
Would be a great feature to have without any doubt, but to be honest I have rarely missed it. Areas that you can't reach with your brush will most likely be areas that will be occluded by other sub-objects and won't be seen in full detail anyway, so painting those areas in 2D view is not a big problem, at least in my opinion.
Nevertheless, being able to hide polygons based on connectivity/ selection sets/ material assignments is a quite obvious feature that should have been there from the beginning.