Maybe ENY could vary by field size - larger fields naturally have more aircraft type availability than small fields - so as ENY begins affecting a country, the smaller fields feel the pinch before the larger ones do. And what if you could affect the ENY for a field by flying cargo to it? Spend enough time bringing in cargo loads and lower-ENY aircraft like P-51Ds could be activated at that field.
'course, you'd probably want a bigger, faster transport aircraft than the C-47, so you could do it faster or in fewer trips.
Maybe THIS beauty?

The quickest way to avoid negative ENY effects would still be to change sides, but for those who simply cannot fly or fight for any other country but their own, this would give an option to gain some control over the situation. Instead of logging off if your plane isn't available, you'd fly a cargo flight or two to get what you want.
Another great thing to do in the sandbox. And opposing fighter pilots wouldn't be opposed to seeing more cargo planes in the sky, I'm sure...