I'll outline the procedure here but be aware that enabling this can cause a performance impact on the server.
What you have to do is create a account that will host this mail folder. Then via IMAP directly or imscripter send the commands to set the proper ACL on the folder.
It's not neat but it will work. The other trick is you will have to tell your users to subscribe to this folder as it will not be done automatically.
You might want to consider an alternate solution as well, such as a NNTP server which is designed with this sort of feature in mind.
iPlanet is working on extending the functionality you're looking for in a future version from what I'm told.
-Chris