This functionality would aggregate a lot of value to the RadTreeList.  I will explain you why it is important. In order to prepare the information to give it to the TreeList, it is necessary to make a lot of hierarchical calculations that consume a lot of CPU and memory resources.  It causes that any server could be busy doing the same complex calculations with each click expanding any level. If you have a lot of users that expands a node of the tree, the server could crash. I prefer,  RadTreeList had all the information one time to prevent making unnecessary recalcularions with every click. 

I like Telerik components, but this issue make me to search another tree component in the market.  Please reconsider this functionality in your next version of your RadTreeList.