I have done something similar to that using DNS entries to determine internal/external access. The users wanted to force Self-Service access to all users when they logged into PeopleSoft if they were at home, but give the users full access if they logged in via VPN or at their desk. That way, power users would not have full security unless they were authenticated via VPN or at work.
We accomplished by customizing the USERMAINT component, created a new security table for "self service" security, adding custom Signon PeopleCode, and setting up a 2nd DNS entry that was exposed for external access.
To start, the network admin set up two sets of DNS entries that pointed to the PS web servers. For example, psoft.company.com was the internal DNS and only set for local network access, and selfservice.company.com was the external DNS available for global access.
Next, we customized the USERMAINT component (User Profile). We hid the delivered Roles tab and replaced it with a copy. The new page was pointing to a copy of PSUSERROLE. We did this because we want to use the delivered PSROLEUSER table as the "current" security table. If the Security Admin wants to update a users internal (or day-job) security, we want to store if for long term. (This will make more sense below). So the new custom table, say XXROLEUSER, is now the master table for security.
Then, we added record/page/component for the Security Admin to define "self service" security (XXSSROLES). This record/page just stores Role Names for when users log in externally. This allows the Security Admin to update self service security for seasonal changes, like Open Enrollment. The admin can add a role during Open Enrollment to grant additional access, and then remove it when the period has ended.
Last, we created a custom Signon PeopleCode step that handles the internal/external security switch. The Signon PeopleCode grabs the URL used to log in. The code parses the URL to see with DNS entry was used to access the system. If the DNS entry is "selfservice.company.com", the code will clear our the rows in PSROLEUSER for the user and insert the roles from the XXSSROLES table. If the DNS entry is "psoft.company.com", the code will clear out PSROLEUSER and insert the roles form the XXROLEUSER table.
This code change turns PSROLEUSER into a transaction table. Because the Tools system relies on this table for many different security functions, it is important to create a copy to use as the "master table" for use in the User Profile component.
If this is something you are interested in trying, I'd be happy to post some sample code for the Signon PeopleCode.