Well my friend, I have good news for you!
Last night I spent a good deal of time playing with the various HDMI and CEC/ARC settings on both devices, suspecting this may be a handshaking issue. The good news is, I found a fix!!
Here is what fixed the issue for me: I went to the video output settings on the Shield, and began cycling through the supported display modes (I believe the Shield determines this by polling the monitor for supported resolutions/refresh rates/colour depths). I changed the default choice (which the shield called '1080p @ 59.97 fps - recommended') to 1080p @ 60fps, and the dd+ stream was immediately detected. By switching back and forth between the video settings, I was able to repeatably replicate/remove the issue.
Obviously this may not work for everyone, as others will have different display devices. For the record, my display is a Panasonic Plasma Model#PN60F5300BFXZC. Even so, I do believe it may help some people, because it shows that the NAD can be picky about the format of the video stream accompanying the bitstream audio. I will do some more testing tonight, but for anyone having similar issues, play with the refresh rate/color depth/resolution settings on your video source and see if it helps.
Now, in the process, I have found two other issues, but I will document those in separate posts