So, I've boiled it down to a simple case that I think captures all the important parts of my implementation. There's a commented section in the code-behind that can be uncommented to enable Load on Demand. I didn't supply any of my futile implementations in TreeView_NodeExpand because none of them have worked. I'm asking for what might go in there to make this example work.
My aspx file is:
And my code-behind is: