Use of the pommel is a nice feature of Conan combat.
Normally it is used when in very close proximity to the opponent, after having parried and one's blade is directed backwards. Usually it is part of a trip attempt. If successful it is all the more easy to deal a final stroke once the bad-guy is on the ground (=prone, -4 Defence).
I also have house rule where the PC's may take the sword by the blade and deal a bludgeoning attack with the guard. I'm still trying to figure out a good AP and DAM for this kind of attack...
The guard should have some AP. Historically, the manoeuver was done perforate armour or a helm, thus wounding/killing the occupant. Some swords had rather pointed guards (termed Gaddhjalt in Norse countries, IIRC), perhaps for this purpose. A few old fencing manuals show combatants using this move. It was called Mordschlag in German sources, which means "Murder Strike."
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.
Maybe what you could say is that it affords of AP itself as an attack, but allows the wielder to use his STR bonus as AP in an unarmed strike (normally an AP0 attack and, therefore, normally unable to use STR bonus to affect AP).