Ahh, I see now. Well, I have the Kensington FM/RDS transmitter for mine. The only external controls for mine are selecting the FM transmission frequesncy, which gets displayed on the iPod sceen (only in iPod mode).
When I tried stable v3.1, only charging worked. But once I upgraded Rockbox to current (r20102-090225), it is functional. The only features that still don't work is displaying the FM transmitting frequency on the iPod display (no surprise, no big deal, since I have 3 presets saved in the transmitter anyway), and displaying track name and track artist on the radio, via RDS.
If you get the device and test, posting the details about what exactly doesn't work with it would be the place to start. I suppose....