Today we will be releasing an updated version of the MOSS RadEditor (5.1). I have already sent you instructions on how to modify the editor dialogs in RadEditor for MOSS 5.x. The difference with the new version is that it will allow you to modify the file browser dialogs (Image, Flash, etc. managers) as well.
As for the bug with the editor dropdowns in full screen mode - I just tested and it has been fixed in the new release (5.1).
All the best,
the Telerik team