I've got a good start at figuring out the protocol. Rather than post the details here, I am going to post my findings to my website at the below address. There is another thread here with people looking for the protocol and I don't want to have to update multiple posts when I find out something new.
http://andy.dynamicbits.com/hdradio/
If you have any questions, please contact me via my website, or post a reply here. I am subscribed to this thread.
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Andy
Additional keywords: Directed DMHD-1000 TI DRI350