Good point - all of the 3-engined-ferry speculation appears to be based on the Reuters article from a couple of days ago, quoting unidentified BEA sources.
Given that the aircraft won't be flying anywhere unless and until it has been established that there is no airframe damage that would affect the safety of the flight, that doesn't rule out fitting a new, operational engine (and probably a new pylon) to allow a 4-engined ferry.
Strictly speaking, provided the measures taken to recover the aircraft to France don't compromise the investigation, how and when it's done is none of the BEA's business, it's up to Air France, Airbus and the regulators.