A recent Opinion by the FAA Office of the Chief Counsel, changes my understanding of when you can file an IFR flight plan. I had been under the impression that both the plane and a pilot had to be IFR rated to file and fly under IFR, but his opinion states,
Your fIrst question asks whether the prohibition on operation of a piston helicopter under Instrument Flight Rules (IFR) also prohibits the filing of an instrument flight plan with the notation “VMC only” in the remarks section for instrument training purposes. The answer to your question is no, an aircraft does not need to be IFR certifIed to operate on an IFR flight plan, provided the aircraft remains in Visible Meteorological Conditions (VMC). See §61.65(c), §91.205(d), and FAA Order 8900.1, paragraph 5-439. However, we caution that the pilot in command must ensure compliance with the flight rules and not to expect special handling from Air Traffic Control.