I think the following will work.
acs-mail-lite is the boundary for processing inbound and outbound email. Outbound notifications and acs_send_mail send mail via acs_send_mail::send. All inbound are handled via acs_send_mail.
Intercepting outbound message_id and replacing with the new paradigm will work for new cases.
Other implementations can then continue unchanged thereby limiting risk for affecting any existing deployment.
