i dont know anything about rockbox's code but from my observation it seems like the software turns off the ipod based on voltage/battery %, i know when i used my ipod it would run till the hardware nearly quits, and i'd start to see the LCD flicker, the rockbox just runs solid and then suddenly goes off saying it ran out power.
Glad to see someone trying to get the firmware to be more efficient, it really eats up power. If we can get power to be as good as the original ipod's or even better, then rockbox will be unanimously the most competitive thing we got to the ipod's limited firmware.