Electrical wiring/grounding - continuity via metal junction box?
There is nothing in Code that says that a metal box can't be part of the fault current path -- this is done all the time in systems that use AC, MCI-A, or metal conduit.  (Personally, I'd use a self-grounding receptacle to save the ground clip and pigtail for neatness' sake, even though it doesn't get scored against box fill.)

If you are curious, btw, look up NEC section 250.148.