I'm not sure, would have to think about it some more, but creative thinking and adaptation (like you have shown) are definitely part of the solution to what so many folks see as a problem.
Speaking just for myself...I would not like to be restricted from joining any country. I would see that as what many folks who are complaining say it is, limiting my choice of playing where/what/how I want to play. As long as someone, in your scenario, is willing to suffer the ENY then who's to say they cannot join rooks?
Now of course in your scenario those already playing rooks would be penalised for the new guy joining the side with the numbers, As long as they were allowed to switch at will I would not think this to be a problem. When you land and exit your La7 and find the ENY kicked in while you were on sortie and you cannot reup in another one, you should be allowed, in my opinion, to switch sides and fly the La7 for another chesspeice.
I have heard that unlimited or loosely limited side-switching will lead to espionage, but don't kid yourself, spies are rampant already and will continue to be so even if side switching were limited to once per tour.
I don't know the solution, but I think that given enough time the current mechanics will work themselves out. I mean everything folks are complaining about, EVERYTHING that is perceived as affecting them negatively, is only doing so because of their own personal choice. If you have some sort of rabid loyalty to one of the chesspeices that prevents you from switching sides when your team outnumbers all opponents, then it is YOUR choice to only be able to fly the P40. If you are afraid to join a side that is outnumbered, or are afriad of not being on the side with numerical superiority, then it is YOUR choice to be restricted from flying the uber-ride you want.
Once it sinks in that personal choice is the root cause of all these perceived injustices I expect the whining to die down to normal levels.
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...so you were a MAW eh? that splains a lot....lol...Hubs was a BOP once too...