I think where the confusion lies is in how many CD players use the iPod dock connector. The connector on the bottom of the iPod (and Sansa) is not just USB.
This CD deck is using the Sansa as a generic UMS device and providing power, no custom Rockbox code is required, he simply wants to charge via USB and line-in the audio with only one cable.
If you look at the player's port you'll see there are twenty-some pins. If you can find a source for Sansa dock-connectors (rumor is a slightly modified iPod one fits) and a pin-out you could construct a custom cable which is USB power and a line-out, and only splits the two in the last six inches so you dont have either a mess of cables or the hassle of changing volume so your headphone jack is at line-level.