The thing is, every time I throw a punch, or a kick, or stab with a knife, it can be argued that I am making a called shot, as I am aiming (at that moment) for a particular location. I am not just blindly swinging in hopes of hitting something (of course, I hope to hit.....). Now, that location is generally a target of opportunity, but once I am thrusting or swinging, it is for a location.
RMs combat system is an "after the fact" targeting system, where after the results are tallied, the location that became open prior to the strike is determined. (I guess it could be called a retroactive targeting system.)
But, not taking into consideration the concept of called shots, just because they are harder than regular attacks, is a miss in my book. But, then again, I have no problem in doing it my own way anyway. Something as easy as a -50 to -100 OB, depending upon the location shouldn't be discounted. (I think those mods are a little high - I would go with -30/leg to -100/eyeball, or something like that.) And remember, they still need to us OB for their own defense, so this is just another thing cutting into their OB.
Also, I think it is totally reasonable to require them to develop Combat Skills that could/would support it, like Disarm, Feint, Swashbuckling, etc... Maybe saying that the maximum OB they can devote to a called shot is dependent upon that skill bonus (before the mod).