I was briefly looking at doing something similar a while back. I believe it would involve either modifying or replacing fmtxd. The problem I came to, and my understanding of RDS may be less than perfect, is this: You tune a frequency, it uses AF to transmit a list of stations linked to the current station. The radio occasionally checks the other frequencies and if it is stronger than the current, it retunes.

The problem is that if the transmitter changes frequency before the radio, it loses the signal including the AF data. But if the transmitter changes frequency after the radio, the radio won't change because the new station's signal strength isn't as strong. In my case, with the way my car radio (Clarion RD3) works, I could get by with just up and changing frequency without AF, because if my radio loses the signal altogether it will quickly scan the other frequencies and look for a station with the same PI code. So on my radio, I could just change the transmitter frequency and not update the radio, and it would find it. I'd love to hear other views on this though

I also wanted to look at using AF to transmit the frequency of my local BBC station, and set the flags for it to use EON. Theoretically, this would get my radio to watch the BBC station and, when they start broadcasting traffic (TA), the radio should switch - like it does when listening to another BBC station. This way I can listen to internet radio and still get traffic reports, but I haven't had time to do much work in this way yet.