There are several things you might check:
1. Customizing the RadTreeView appearance at the RadControls documentation.
2. Creating a control adapter for the RadTreeView, so it renders in the way you want it.
3. Create a custom user control using the Sitefinity APIs. This way you would have a full controls over the output and the control representation.
As for the filtering, you can place the filter in the EventsView control itself. This way, the CategoryTree control will simply append the filter to it, rather to remove it (and show all items).
Let me know if I am missing something.
the Telerik team