Let me preface this by saying I'm already certain its an unworkable, non-starter of an idea that HTC would never implement. I think it probably violates more than one of HTCs design philosophies about AH. But, its not impossible to implement codewise, and it does expand the slots for planes that have too many skins for them, without adding even one new aircraft model, so I thought I'd share.
I don't know if its ever come up before - my guess is no, because it violates the "everyone has access to everything" principle of AH's game design (I'm ignoring perk costs and side-balancing ENY limitations here).
Simply put, you could create a country ownership variable for skins. For the majority of aircraft skins, the value could be '0' meaning the skin is valid and usable by all three countries. But for aircraft crowded with skins like the P-51D, you could divide up the skins by fighter group, i.e. all 357th FG P-51D skins get value '1' meaning they only show up in a Bishop player's skin selection list. The 352nd FG could be '2' for Knights, 4th FG skins get '3' for Rooks, and so on. In the hangar, each player only sees the skins that are valid for his country (plus those that are valid for all countries). I came up with a list of 22 FGs that operated the P-51D last night, and a master list could be created that spreads the most popular and colorful FG skins equitably among our 3 countries, and evenly divides theater of operations so each country has some groups that saw service in the MTO and CBI and Pacific as well as the ETO.
I think one result of this division could be a fostering of esprit de corps within the ranks of each country, from having rights to certain skins that players from other countries can't use. And the flip side might be competition and rivalry fostered by having groups of players in different "uniforms". I imagined witnessing a field attack by a group of Blue-Nosed 352nd 51's of the Knights, or fighter sweeps conducted by the yellow and red checker-nosed Mustangs of the Bishops' famed 357th FG. There would be so many slots available you could conceivably do 2-3 skins for the really popular FGs. And a flight of Mustangs, each with a unique skin but all sharing the same FG scheme would be a great sight for films and videos. You could actually set up a "Bottisham Four" flight of Rook 361st FG Mustangs, for example.
I was also thinking about the future, with AH probably going on Steam. If AH gains popularity and many, many new players sign on, it stands to reason some of them are going to be interested in skinning. And a few no doubt are going to be extremely good at it; capable of creating fantastic looking skins. And what do you think their reaction will be when they learn that the P-51D's slots are full; that no new skins will be accepted for it, period? As far as I know, AH is the only flight sim that controls and limits skins in such a centralized way. I understand the reasons for it, but I suspect that any really talented skinner who comes here as a new player is not going to be very pleased with that situation...