For example I could rent out (subject to appropriate insurance of course) my N-reg plane, but I must not be flying in it with the renter because then it would be PPL Cost Sharing
Just curious. If the renter is legally acting as PIC, then you are just a passenger along for the ride and you are not in any way using any pilots license to claim any privileges/hours. I would assume that that would mean that PPL Cost Sharing regulations do not apply.
I mean, if I'm a regular passenger aboard a commercial aircraft, the rules, regulations and economics of the flight don't change one bit with the fact that I accidently hold a pilots license or maybe a share or two in the airline, do they?
By your logic, the renter can take any passenger he wants, unless the passenger accidently holds a pilots license, or accidently is the owner of the aircraft. That would be odd.
I do agree with having to maintaining the aircraft to a possibly higher standard than what would be required for a purely privately-operated aircraft.
BA, I see you're based at EHLE. It might be worth checking this with Bert Huizinga from Wings. I know of at least one aircraft in his hangar that's privately owned but rented out occasionally via Wings. Perhaps he knows more about the legal issues, and who knows, he might be able to act as an intermediary for you.