NEC 2014 Article 400.7 (A)(11)
Between an existing receptacle outlet and an inlet, where
the inlet provides power to an additional single receptacle
outlet. The wiring interconnecting the inlet to the single
receptacle outlet shall be a Chapter 3 wiring method. The
inlet, receptacle outlet, and Chapter 3 wiring method,
including the flexible cord and fittings, shall be a listed
assembly specific for this application.
AFCI protection would come from the circuit and receptacle the extension is connected to. Tamper proof: I don't know if it's necessary any more than an extension cord (which it really is).
-Hal