i believe phone manufacturers all over the world doesnt get what you want. all cables i can think of charges.
i dont think usb charging can do that much damage to your GY or your batt. i found some articles even saying Li-Ion batteries actually do better with frequent partial charges/discharges. it will in fact have more benefits, ex tethering which requires power.
if you dont want it to charge while transfering or syncing files, you should use blue tooth.
if there should be an app, i coudnt think of one. id be happy to know if something exist but would leave that to the market for somebody else to download.