It’s not necessarily just a matter of firmware, there has to be controlling circuitry in place to dim an LED. Since there seems to be only two states for the battery LED (off and on), there may only be on-off switching circuitry in place for the firmware to control. If that’s the case, it will take a HARDWARE upgrade to be able to control the brightness of that LED.
(the circuitry providing this functionality must have already been in place for the buttons, which I suppose are a separate assembly part)