We deploy our client via NAL. During the install we create the folder that we want the caching mailbox stored. We then put in this registry entry-
[HKEY_CURRENT_USER\Software\Novell\GroupWise\Login Parameters\Path To Cache]
When they create their caching mailbox, all they need to do is select the defaults.
I know this is not exactly what you want, but it at least gives them a default location.