Thanks Kirk.
Here's the reply I got from Topaz (which indicates a small jpeg should be in the file):
"We don't test or support anything in particular with PhotoMechanic, so I'm not 100% sure what might be happening, but in other places were a JPG preview is needed, our files will show one. The dimensions are also included in the exif data. Usually 270 x 180 or so. It varies. As far as controls, there are no particular settings within the program itself to control this behavior I'm afraid."
They asked if I had another program I could test it on and unfortunately I don't. I may just have to settle for using a jpeg rather than a dng.
pj