I assume you're looking for a sleep mode similar to Apple's firmware?
If so, then that's different to just sleeping the CPU/COP, which Rockbox already does. A sleep mode involves clocking the CPU to a very slow frequency, and powering down all the other parts of the hardware.
IIUC, the developer working on the PP5002 ipods has successfully tested such a sleep mode, but has not yet integrated it into Rockbox.