I've figured out an easy way to do this now, so no need for a reply.
For reference, I have done the following:
''' <summary>Used to allow the class to internally select an item. Not for user use</summary>
Private m_AllowSelect As Boolean
''' <summary>Cancel the selection if a user tries to select another node in the tree</summary>
Private Sub RadTreeView_Selecting(ByVal sender As System.Object, ByVal e As Telerik.WinControls.UI.RadTreeViewCancelEventArgs) Handles RadTreeView.Selecting
and where I need to set the selected item internally....
True 'temp allow select
False 'disallow select
This means I can set it programatically from the class, but the users cannot.