Having used an iPad for both stills and video and having found it barely adequate, clumsy and hard to aim and hold still, I can't imagine how an addon could improve the N7.  It could make it clunky and hard to handle, and goodness knows what potential there could be for battery and perhaps even circuitry problems. 

I'll pass.  If god had meant for the Nexus 7 to have a rear-facing camera, he wouldn't have put one on the S III.