Asking for this out of pure greed.
PnP thinking: "Bow Strength" doesn't exist, it's a property of the "mighty composite" bows that allows strength bonus to ranged attacks. In other words, not limited to a specific class.
DDO thinking: Rangers are mostly a stack of predetermined bonus feats and a small unique spell list. Diehard is available as a seperate feat. Very little they get is not available elsewhere, so why should bow strength be a uniqueness?
Pure greed: Want to build a pure fighter, deeply range specced (elf, +4 to hit, +6 damage with any bow, at any time), to play with the (sorta leaked) Kensai PrE.
Alternately, coding in the mighty composite ability, much like a special material (doesn't add to enhancement +, as it's a physical property and not a magical one), would work, just not as nicely for non rangers (capped at +5 Str bonus to damage).