i have never tired it with rockbox as i never charge in rock box BUT...
check out
anything but ipod forums and check out the section on sansas.
i remember reading a whole post on glitches in the sansa firmware such as getting rid of those random files you cant delete, and using the player as it charges.
it has to do with the record button and some sequence that confuses it.
i am trying right now with mine but to no success.
you can also buy a sansa care kit off ebay for 9 or 10 dollars that comes with 2 cases a wall wart to charge the sansa and a cigarette adapter. the wall wart should charge it but not make it say connected.
btw if you didnt know. a wall wart is just a transformer. this one steps the 120V down to 5V for your sansa.