A houserule that my table plays with (harvested from someone else on this board) is that when you highlight a Battle-hardened character's Cool, you're actually highlighting Act Under Fire.
What about all the other moves – even custom moves – that use Cool? You get no experience when rolling those?
So when he Acts Under Fire, even though he rolls Hard, he gets an XP because Cool is marked. It keeps stat-sub moves from messing with what I think is the best part of highlighting, which is incentivizing behavior, at the cost of being slightly less intuitive.
It seems the simplest way to do that is to highlight that character's Hard, if that's the stat they'll roll when doing what behaviour you want to see.
Highlighting only a specific move is nerfing the incentive, I think. Stats are broader than specific moves, and highlighting stats is meant to cast a broader net than a few specific moves.