Is there any method to secure the possible values a user can enter into the "external_os_user" field?
Workload Automation AE 11.3.x
At this time EEM can be used to secure some but not all fields. The field external_os_user is not secured by EEM policy checks at this time. If you want EEM to secure a field it does not currently please post your idea on the communities as an enhancement request for product mgmt to consider in a future release.
Additionally you might consider using the autoextvj utility to secure the field.
Here are two links to videos that show examples.