In Circulation::SendCirculationAlert we find the only use of the C4::Message module and one of the last TRUNCATE calls btw in the same context:

        my $message = C4::Message->find_last_message($borrower, $type, $mtt);
        unless ( $message ) {
            C4::Context->dbh->do(q|UNLOCK TABLES|) unless $do_not_lock;
            C4::Message->enqueue($letter, $borrower, $mtt);
        } else {
            $message->append($letter);
            $message->update;

The append method digests multiple notices by looking for a ---- separator:
    if ($letter->{content} =~ /----/) {
        my ($header, $body, $footer) = split(/----\r?\n?/, $letter->{content});

In the first place I would not mind sending these notices separately. But anyway.
If we do combine them (within the timeframe of the message queue processing interval), it would be convenient to allow separating too on an html tag like <item> as used elsewhere in notices.

I looked a bit at the notice digesting and actually only find this feature working for advance notices. The overdue digest flag is a bit weird: the job already digests by default?