Instrumentation amplifier with frequency-dependent gain?
Can't think of a reason it won't work.  The AD8421 uses a standard three op amp topology.  It isn't one of those wacky ones designed to have a fixed gain when you leave RG open and another fixed gain when you short it.  It should just work, and it shouldn't hurt the CMRR of the output stage of the IA.  Give it a try, and kindly report back!