The easiest way to deal with this, would be a separate bootstrap for the Claw (wClaw) for each claw on the Hero. Then on the ability, have a drop down to select the weapon to apply the bonus to. So if your monster has 4 claw attacks, you would add wClaw to it 4 times. (Again, this is NOT the way that monsters are built in things like the Bestiaries.)
Even the Eidolan like you mentioned does not do this. Each time you add the claw evolution, you get 2 claws, not 1. Meaning that evolution bootstrapps wClaw to the Hero (Eidolan) with the tag Value.2 on it. So you still could not select the Right claw, and not the Left claw.
This way might be a little easier than the way that Shadow was mentioning, but his way will give you more options for control.
Even the Eidolan like you mentioned does not do this. Each time you add the claw evolution, you get 2 claws, not 1. Meaning that evolution bootstrapps wClaw to the Hero (Eidolan) with the tag Value.2 on it. So you still could not select the Right claw, and not the Left claw.
This way might be a little easier than the way that Shadow was mentioning, but his way will give you more options for control.