Hi Simon,What do you mean by using RadEditor for MOSS as default editor for the WSS site?
You can use the RadEditor Web Part as well as use RadEditor in List Items. The following two articles provide guidance how to:
Using the RadEditor WebPart,
Using RadEditor in List Items.
Please, note that RadEditor cannot be used as a Content field in Web Content Management
scenarios in WSS v3.
the Telerik team