I can find an answer to this one.
So I have to go from stencil to texture to alpha to mask?
You guys like doing everything the hard way, don’t you? Why not just have the stencil work with masks?
hey Reactor, I agree that it would be handy and more direct to use the Stencil to ctrl-paint a mask in edit mode – maybe in some future version of ZBrush (?)
I imagine that programming-wise this might be related to the mysterious rumors and possible gestation of displacement mapping
However after some experimenting with 1.55b, even after a couple repetitions of Tool->Deform->Divide+Smooth (to increase mesh resolution), I see that for masking currently we would get back to the difference between vertex painting (painting polygons) versus the finer detail that texture painting allows
BTW Matthew: that was useful info in the new manual about “standard rule of thumb in the graphics world” being about 20% modeling and 80% texturing
I’m sure I have seen some games where it was more like 5% modeling and 10% texturing 
[stuff about Selection functions moved to a separate topic]