Hi James O'Brien,
The requested functionality is not offered out-of-the box by RadEditor, but you can implement your own custom dialog
I have such an example for RadEditor Classic (RadEditor.dll) and for your convenience I attached two projects that you can examine and use the idea when implementing your own custom dialog for RadEditor for ASP.NET AJAX (Telerik.Web.UI.dll).
the Telerik team
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 Public Issue Tracking
system and vote to affect the priority of the items