From what I can tell, the industry seems to be giving up on AM-HDRadio.
Here in Atlanta it was never widely adopted but, over time, those that did adopt it have turned it off and are back to the analog signals.
If you were to compare the AM radio list on HDRadio.com from years ago to now I think you'd see much the same thing.
If it's like FM HDRadio then I don't really know how it'd work. I'd think that the signal would be even less stable than FM as power lines and over passes do a pretty good job of knocking out AM, and, for home use, it's really hard to get AM into the home.
What's weird is that I remember when I was a kid we regularly listened to AM in the home and, yes, sometimes it helped to turn your radio a little bit, for the most part it worked quite well. You don't get that now. I don't know if it's because of more electrical devices and interference, crappier AM radios, or a combination of them both.