Hi Philip,
I updated an almost unknown piece of code written by Penny Leach which was importing parent/mentor role from an external DB. The original version was only compatible with Moodle 1.9. The updated version of this plugin is available here:
https://github.com/maxCSA/moodle-enrol_dbuserrel
Be careful because I haven't tested it a lot. I need to polish it a little bit as well
Regards