Here is the response from Support:
We have a sample showing how a hierarchical taxonomy could be queried and displayed in a RadDropDownTree. I think you will find it useful, so I have attached it to this ticket. You can use it directly or modify it to bind to a tree view the same way, since the datasource is shown how to be queried. You can find how the URLs to filter the widgets could be constructed in this blog post - construct URL section.
I hope you find this useful. Contact us if we can assist you further.