I created a simple .tgd file and examined the format to see how to generate additional nodes outside of TG2.
After generating several heightfield nodes outside of TG2 and adding to my .tgd file, the nodes show up nicely in the node display. However, when I click on the related shader in the node list, there is no shader preview. There is no bounding box in the preview window, though there is a bounding "point" (I can mouse over it and get the correct name).
If I manually add a heightfield within TG2, using the exact same image (100x100 flat grey tif), it displays nicely in the shader preview and shows up in the 3D preview (and ultimately renders correctly).
I'm probably blind, but I don't see the difference between what I generated with my program (node names start with HF) and the one I added by hand in TG2 (starts with Manual).