What I did was I took HDARTISTX and HDTITLEX and added 1-6 to my HD Radio screen to see which one was displaying info. I decided to just create buttons for the sub channels so I could run the HDSUB;X commnd to change the channel. If you use the code that just cycles through the sub channels from 1-6 it will hang.
Also, you can hook up the visteon display and it should show you which sub channels have been detected.
I'm guessing the visteon head unit just scans each sub channel to see if it has active content and only gives you them as options.