Interesting that Jack Henry is named in the README! The vendor we are dealing with is Jack Henry. They are a provider of Banking software. These printer/devices are configured in their AS400 software and the documents using the 'overlays' are 'requested' from within an IBM Client Access session.
I have asked Jack Henry to shed some light on your question "in what way their 'software overlays' are different to normally accepted macro standards" I will forward their response when received.
Truly appreciate your help and research.
(Still looking for the 'not a solution' button:))