I agree with Hoshikogi, the damage reduction would have to be massive, and should happen (I wasn't clear on this part so ignore me if it was already intended in your description) even if the player behind the PLD is not currently at the top of the hate list.

Any tank worth his salt shouldn't be taking hate anyways unless your DD (and I mean BLM only here, nothing else can spike like it in a short amount of time) wants to actually take hate. I myself havn't had any problems with hate tanking on PLD and WAR (ifrit here, since he seems to be a stable constant) over WHM hate, LNC hate, and most of a THM's mana pool, usually only one chameleon from a THM can let them burn all the mana they have without ripping off ifrit (ofc BLM could do it due to a nearly double mana pool from vert, but once again, they can easily hold back for a bit).

If even 1 DD could be able to safely DD from behind the tank, MNK would see action in more events (yes I know they're awesome DPS and can still do any event well, but with a heavy cover such as that any risk involved with taking a MNK over a THM/ARC would be cast aside.)