I have been struggling with this as well some time ago and thought this was not possible. Since it is not possible (I think) to determine if the connection is coming through the internet or intranet. So didn't pursue this anymore. It was 'solved' by assuming that if you have access through the internet (they would first need to logon to the intrnet) then they'd have access to PeopleSoft as well.
Hope this helps and would stand corrected if this was somehow possible.