ZBrush (and it's free companion, Sculptris) are great fun, and are invaluable programs for making things for higher-end games, but polysculpting for a game like Torchlight is complete overkill, and shoehorning it into the pipeline would very likely be more trouble than it's worth. (Which is not to say you couldn't try to, if you really wanted.)
Given that, I think Nemesis might just be sculpting that gargoyle head for fun, rather than for a mod.