Since acs-messaging is being phased out, would this work to add optional message logging in acs-mail-lite?
ecommerce has a logging feature for outbound messages that will need to be integrated.
Your suggestion would fill the ecommerce[1] audit features/requirements and could easily be extended to whatever needs audited within an implementation.
acs-mail-lite has message logging with the mail-tracking package, which works out of the box and has some special features that you would not want in a general audit package like links to the persons who recieved the mail, display of mails, forwarding of mails. But otherwise I agree and great that you like the idea.