I can't speak for zbrush -- my understanding of the app is limited to a prior version where that's not possible -- the new one, though, is really more fully featured. IF there is a program that can do it "easily", I'd bet it was one. I can say that even Deep UV can't do it. lol I don't currently have the free version installed, so look under the tools -> UVs menu for Weld and stitch (they may not be needed in this case). The only other really important features are point editing and the zoom tool. The point editing changes everything to a bunch of dots where the lines intersect (the lines are vertices) and lets you move them around individually or in groups. as far as processing them bit by bit, if you mean can you do it over time, saving to a new file, then hell yeah -- and the ability of UVMapper pro to hide unselected stuff is a huge help there, but isn't critical. When you are doing it, you'll have to do it a bit at a time -- aside from the eyestrain, it's really, really, really boring stuff to do. If you mean can you do "only" the hands and not the rest, then yes as well to an extent -- you can do a artial remaps of any figure (I've remapped Posette's hands to take higher res textures for a close up series on making change). Mapping, all by itself, is a really cool thing, that takes a weird mindset. But you can do some really awesome stuff with it if you plan things out properly.

thou and I, my friend, can, in the most flunkey world, make, each of us, one non-flunkey, one hero, if we like: that will be two heroes to begin with. (Carlyle)