Why don't you just "reskin" the fluff text of armour? Instead of calling it "chain mail", just call it "superior training" in your game and don't let folks simply buy it? The game doesn't really care why your AC is 28, just that it is.
Ain't nuttin' wrong with that!
It could be a Feat, for instance...one that scales with level or one that has a tree of dependent feats that boost its benefits.
It could be an ability gained via a PrCL.