Using ArgoUML and ArchGenXML, I created a special type ImportedProjectsFolder to hold items of type ImportedProject. I did this by just giving it a stereotype of <<folder>> or <<large>>. Although this did give me the required folderish type, Generic Setup did not want to create the folder: on installing the product, it said:
('Could not adapt', , )The solution was to not use a stereotype, but to use a generalisation from ATFolder (<<stub>>). This way, the ImportedProjectsFolder inherited the required interface implementation, and installation was succesful.