Because of the price of the charger, they probably figure it's expendable; you probably have nothing to lose if you open it up. Will the radio accept a regular wall wort type charger? If so, you can see if the battery will charge with it, run it down, and recharge it again. If that works, get out the iron and screwdriver.
ETA With Kenwood, the service info for accys are in the service manual of the radio(s) the accys are used with. I don't think Yaesu does that, but you might check.