Except she could wear heavy armor. I always considered celes to be more of a PLD than anything. Although since she had native damage spells she'd be more akin to DRK.
Either way it doesnt matter. This (the op) is basically just cover for magic damage, and it'd be a PLD ability (possible rdm could get it too though, see shield mastery)
Yeah i'm at work didnt have time to read the whole thing
I dont particularly care about pld getting a new ability, but the way you presented runic in the op is a pld ability, not rdm.If you want PLD to get a new ability, then give them an AOE spell or JA that works like stone skin, but for magic damage only.
After going back and reading the new runic ability I don't really have an opinion on it, it seems too complicated to actually get implemented.