I could be wrong but I don't think there is a way of excluding an accepted skin from a specific arena. I'm just saying that "But you accepted the CAF skins!" will not be considered a valid reason for skinners to submit other non-WW2 aircraft for MA use.
To be clear I won't accept:- Pre-war, post-war, training (unless they likely saw combat), unarmed formation ship or recon variants, neutral air forces, prototypes or test planes, captured aircraft or tanks (maybe if used in very large quantities, i.e German T-34s). I'll also not allow skins on variants of the ride that look completely different or have totally different performance to the subject modelled by HTC. These are the rules that Pyro and Skuzzy applied to MA skins in the past so unless HT tells me different, that's what I am sticking to. And yes I am aware of the skins in the game that break these rules and that have have slipped through the net in the past. Personally I would like to delete these from the game but its probably not worth the hassle this would generate.
I'm happy to argue the case for any marginal skin subject though and if the skinner doesn't like like my ruling then I guess they can then take it up with HT. However to avoid frustration I'd suggest that the skinner does all that before creating their skin, not after.