The problem with diesel is the possibility of low temperature fuel waxing in the airframe fuel filters; so even though the engine can safely burn it, it might not be approved.
It may be that an aircraft with heated fuel filters is safe, if not not allowed. Certainly the Puma HC1 didn't have heated fuel filters (so no diesel approval and FSII needed for Jet fuel), later Agusta helicopters do and it's allowed by the RFM.