Why when masking with lasso (or rect/circle) on a thick mesh, does it mask on both sides of the mesh.
Backfacemask turned on does not help. Symmetry is not on.
This is by design. All the marquee tools will “select through” in their operation. Backface masking only works when you are painting a mask.
Try hiding the side you don’t want to affect first. Using the advanced polygroup options like “group front” can help you quickly establish polygroups that make hiding sections of the model very easy.
Ah ok, it was driving me nuts. Thanks for the tip 
Polygroups options in ZBrush really are very comprehensive and can get us out of lots of tricky situations.