Answer
Rather than trying to make the custom-event-driven REST call synchronous and parse Application Messages afterward, route this through a Workflow instead — a workflow's REST/API call step is built to receive and process the response directly, and from there, writing the outcome (success or failure) into a custom logical unit is done through a normal IFS API task calling that custom LU's own projection, rather than needing to poll or query Application Messages to correlate a response back to its request.