Cool Ben, the system I was going to set up in my SoM game was quite similar to your "targeted" attack system. Since SoM calculates each damage type separately (slash, smash fire etc) you can simply rename these categories to "head, R Arm, L Arm" etc. If an enemy attacks with something like an overhead swing, you could have it inflict only "head" damage on the player.
If you wanted to add some randomness, you could make an enemy with 3 identical "Direct" attacks, then use the Parameter editor to "AI odds" and set the first to do only "head" damage, the second to inflict some "head + arm" damage, third to inflict only "arm" damage and so on. I think that would feel more real and entertaining than SoM's default slash/smash/stab/elemental categories. 
If you wanted to add some randomness, you could make an enemy with 3 identical "Direct" attacks, then use the Parameter editor to "AI odds" and set the first to do only "head" damage, the second to inflict some "head + arm" damage, third to inflict only "arm" damage and so on. I think that would feel more real and entertaining than SoM's default slash/smash/stab/elemental categories.