I was checking this out last night and decided to see if Rockbox had done any work on any patches for this problem. Turns out, they had. I can confirm the Rockbox firmware now allows the C200V1 (I have several C240V1's) to connect to the iTrip -- Before, line out on the sansa connector was not working when running the Rockbox firmware. Now, there is a static/fixed volume source on the sansa connector when connected to the iTrip, variable volume on the headphone jack when connected to a compatible charging port, and the unit charges when connected to a compatible (nonUSB) charging port-- All via the iTrip (I would imagine this holds true for any sansa docks as well). Michael was kind enough to commit it to SVN today, but was unable to push it for the 3.7 version in time for release, so maybe in the 3.8 release.
According to the bug/patch board over at Rockbox the FuzeV1, E200V2, C200V1 will now detect docks, charge and supply audio via both sources (headphone and line out).