I would do it as an Item that granted a + bonus to strnegth but only for bow use. Then price it exponitially so you could have a +4 Gloves of archer or bracers of archery that make a users strength +4 when using a bow. It wouldn't make the bow mighty or anything though you could also make gloves/bracers that did that. I would price it close to a weapon versus the gauntlets/belt of strength since all it is really doing is adding damage. So maybe 1/2 to 3/4 the price of a similair weapon bonus. I say 1/2-3/4 becuase damage with a bow is what makes bows weaker than melee weapons by comparison. It is harder to increase bow damage than melle damage.
later
later