Currently we are not planning to introduce such a feature for the RadTreeList, however I will forward your request to our developers for further consideration.
For now you can see if the new load on demand works for you. See the below demo:
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed