Adding new GFCI outlet to existing light switch
Well, there is a way I think:
You can re-purpose the white to be neutral by connecting it to the neutral in the light fixture junction box. This gives you power/neutral/ground in the switch box itself.
Now, how to switch the light? Add a UPB or Insteon light fixture module for the lamp (there are modules that are designed to fit into the light fixture junction box) and wire in a corresponding UPB/Insteon switch in parallel to the GFCI. Now the light is controlled solely by powerline signals.