It is quite common for NPPL pilot owners to take a “full” PPL in the RHS. The NPPL can than fly to the FIR boundary and then allow the full PPL to finish the flight in French airspace. The RHS PPL is on the insurance and qualified to fly the aircraft. Provided both pilots have satisfied themselves that the flight can be conducted safely and agreed what is going to happen I am not aware of any law preventing this. No signatures in the log until after the flight. I am fairly sure that at least one of these flights was ramp checked by the French and all was said to be OK. (I was at the same airfield at the same time this was going on).
What I did not ask at the time was what it said on the flight plan, which had gone “missing” and was the reason for the check in the first place.
Rod1