How can a message template subject in WFMAIL be prefixed by SYS_CONTEXT('USERENV', 'DB_NAME') showin

In System: Mailer workflow, I have copied the Workflow Open Mail (Templated) message and created a custom message template by modifying the subject, adding the name of the instance. I have assigned it to the Workflow Notification Mailer and it works as expected.

The request though is that the subject of all e-mail notifications shows the instance it is coming from using SYS_CONTEXT('USERENV', 'DB_NAME'); so it is dynamically determined. If I create an attribute and assign a value to it by calling a PL/SQL procedure using SetItemAttribute, I would have to call that function in a process. I cannot do it in System: Mailer as it contains only templates and I definitely do not want to modify every single workflow that we use or will possibly use to call this function. So the templates seem like the best option; however, I cannot get