Sculpting Smooth Brush - limit smooth axis

When smoothing in sculpt mode, I often find myself wanting to smooth in 1 direction but prevent another direction from smoothing. Maybe this is possible (idk), but haven't found a way yet. So, let's just say you have a rather thin object, and it's bumpy going in z direction, but you want to maintain the position of the vertices x/y directions (ie, so sides of thin object don't get pulled in with smoothing operation). Is that possible currently? If not, it's something I would definitely find very useful.