We have changed the feature receiver class to handle the "access is denied" error. This way, you be able to activate the editor feature even if you are logged in with an unprivileged account. The new 4.4 RadEditor for MOSS will include this fix.
However, you should know that activating the feature with a normal user will not copy the file. This means that you must either manually copy the file before users start creating their sites or you need to activate the feature with an administrator once. After that you should have no more problems as the file will only be deleted when you retract the editor solution.
the Telerik team