Yes, if you prefer the approach with editing the template of the treeviewitem you will need to do this for each theme you are using. To avoid this you can consider the approach with the ChildrenOfTypeExtensins.
About including the mentioned property in our next release, currently we made our planning and this won't be included. However, if think this is a must-have for RadTreeView you can check our feedback portal
and log a request for such implementation there. This way we will have it logged in our system and we can consider its implementation and approve (or decline) the feature request if we decide that it is reasonable.
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Feedback Portal
and vote to affect the priority of the items