I've been using my reminder emails since quite a while. By now, a cleaner version thereof has been open sourced; in fact, it is a generic mailschedule tool. The basic idea is still the same: at(1) can be used to schedule a task at a given time, and mails can be easily serialised. There is, however, a new idea: instead of telling at(1) what mail precisely to send, we only schedule a generic task at all moments where it might be necessary to send an email; the actual timing information, together with possible prerequisites, is stored in the spool directory. In this way, mails can easily be rescheduled, removed, vetoed, and sending out made dependent on a review requirement. All that is, of course, controlled via email, using a generated authorisation token for external email addresses. The precise on-disk structure is described in the man page contained in the linked code dump.