I've always preferred the idea of the engine supply being independant of the constructors, more or less for this reason. (It's not just engine mappings, but designing components to fit just so from the get go. Mercedes can build an engine to match the car, whereas McLaren has to build a car to match the engine)
Ideally each team would build their own cars and engines as an entire programme, but I don't see that happening any time soon.
I don't really see a solution to Ron's problem though. It would be impossible to police Mercedes making a 'generic' engine mapping that doesn't favour their car over the McLaren car. I'd hate to kick out Mercedes as an engine supplier or race team. You'd have to do the same with Ferrari.