I don't know where ppl are used to gather sweat on Next Island, I guess that Crystal Bay or Papoo Village are good spots. The demand of sweat is true... there's a nice tailoring daily mission that require at least 1000 sweat/day
People have sometimes been gathering sweat near the base of crystal peak on NI (there is a televator next to the tp). The mission requires sweat input, but about half is returned during clicking. People seem to gather at tanzanite tp to do their clicks (mission npc there).