Hi David,
I don't know if this may help you. RememberMe is a block type plugin for Moodle v2.0 made by students of the Berlin School of Economics and Law. It periodically checks if a user has not logged in for a specified amount of time and if so, it sends a reminder email to this user. It was developed as a course assignment, but if you have programming knowledge you might be able to adapt it to suit your needs.
Regards,
Beatriz