> If you have the freedom to define a new list, companion to the
> "conversation" one, you could define for it daily digests (no MAXSIZE
> option, here; no need for notebooks, if you keep them, since they are
> associated to the main list), and instruct those people to subscribe
> there (with default-options DIGESTS). This "daily" list would receive
> mail only from its subscription to the main list. A minor point would be
> the REPLY-TO field: shouldn't the SENDER keyword make the trick, I see
> no simple way to point it to the main list address (and so, an editor
> should be asked to redirect occasional replies to the main list).
 
If you use the Respect option in the Reply-to parameter for the daily
digest list, it will keep the Reply-to that the first list put in.