I am not sure about this can be achieved by setting the property individually for nodes.
Probably, you can set the "TriStateCheckBoxes" property of the treeview to true and chnage the checkbox status from client side for each node, according whether you need Indeterminate state or not.
These links will be helpful in handling client side methods for treeview. Client-Side Programming Basics RadTreeNode
First, make sure you need to use LoadOnDemand. If you have a few hundred items in a treeview it may not be necessary.
Second, if your treeview only needs to retain state on one page during postbacks, you can use PersistLoadOnDemandNodes="true" and your problem is solved.
If, however, you need to setup the first node as indeterminate because you are tracking the user selections in some other form than the treeview, you can do the following but this code will need modification if you have more than one level of hierarchy in your treeview.
1. Set a custom property on the treeview node that you wish to have set as indeterminate
2. Add a OnClientLoad event
3. Loop through the nodes client side and fix the rendered CSS class.
4. Make sure you populate the correct state when you load the treenodes on the server. The RadTreeView will fix the state of the parent node as soon as it's loaded.