Have your API server accept the work item, do any fast preprocessing (validation, etc, anything that is fast and doesn't require a Mac) on it then chuck it into a database and return some reference to the client.

The Mac machines can poll the DB for new items, process them, then submit the finished item of work back to the DB.

The client can poll (using the previously-obtained reference) for the status, or the completion of a job can notify the client (push notifications, websocket, etc) to avoid polling.