Sorry, I don't think this is a solved problem. Here is my phenomenon: I install the latest "Current Build" via the "Rockbox Utility". It seems that the Rockbox USB mode still charges the iPod, but extremely slow and inefficient, comparing to the native Disk Mode (hold SELECT and PLAY while booting). It takes maybe 3 to 4 times more than the native Disk Mode to fully charge. Furthermore, if I try to copy big files to the iPod in the Rockbox USB mode when it has very low power, it is usually forced to turn off because of "Very low power": the power needed for disk spin and file copying is more than charging!
There is no such problems if I use the native Disk Mode. From my observation, the problem of the USB mode is, it charges, but slow and inefficient.