I've found this thread which gives me a clue, but not sure how I would go about it from the BatchPrint business logic.
viewtopic.php?f=26&t=1335
I assume I'd hook into the BatchPrint.ProcessStart? Maybe the BatchPrint.ProcessingLine?
And then how would I
add a handler for the SaveStart of the JiwaApplication.JiwaEmail.EmailMessage object before the salesOrder.Email() method is invoked.
I think I need a head start on this one