Well... the murder stroke also usually assumes that you are gripping the blade as opposed to the hilt. I thought you were referring the to pommel strike more as holding the sword by the hilt and using it as a punch attack. That's what I get for not reading your post carefully.
As for the attack you are suggesting: why not treat it as an attack by a warhammer, since that is what the equivalent would be? Better AP value, but you lose the tremendous damage potential.
And no, I wasn't be sarcastic at all.
Also: I've done a bit of work with historic swordfighting. Half-guards are fun.
It's also why I think that you should be allowed to grapple with greatswords and still cause damage.