I have an Audi A4 (B7). I've been able to successfully sniff out the DIS messages from my RNS-E using the same method that @jamesatfish wrote up at http://forum.canb.us/discussion/91/identifying-steering-wheel-button-packets-an-example (excellent write up!).
I can successfully send a message to the DIS by using the CBT command 02020261484579205345787908. I see the message for a split second before the RNS-E overwrites it by sending a new command ID 0x0261 with the radio station that's currently selected, but at least I know my setup is working.
If the RNS-E is off and I send my same message using the CBT, nothing is displayed on the DIS. Is there a command that the RNS-E is sending to the DIS to "enable" radio messages? Through all my bus sniffing, I haven't been able to find anything specific.