I’ve changed my mind, based on what @erlend_sh wrote in another topic.
Erlend’s idea, if I understand it correctly, adds on to yours by treating reading posts and messages by email the same way as reading posts and messages by web interface. So members would not see gazillions of notifications when logging in after a long period of using mailing list mode. They would also be taken to the correct spot in discussions based on what they’ve already seen by email.
Don’t know if this can be pulled off, but I like it.
Also, as I wrote there: