Well, Zbrush is pretty much unusable without a keyboard, and sculpting programs in general don't benefit from a tablet display anyways. Therefore a cheapo laptop + a small or medium wacom tablet setup seems like a rather obvious and straightforward solution.
Started with the eyes, which are my favorite part, and went from there. C&C welcome, made with my Wacom Intuos 6x8 which just arrived today (first tablet ever).