Make it possible to setup a cron job that can send a password reminder to the user before the password expires.
Document it, in the manual / FAQ / install doc.
"How to setup password reminders in cron (unix) / at (Windows)"
I think the key thing for something like this is I would prefer to work with someone with a present need so that we get the use case right at least for somebody. If you have a user with a present need for this, let's work together on getting this set up and documented (I can provide overview of approach and some help free of charge).