Hi Dave,
There are some plugins related with avoid the enrollment in the courses, maybe this can help you :
https://moodle.org/plugins/enrol_auto
https://moodle.org/plugins/enrol_autoenrol
Sorry if my question is not appropriate, but, how did your resolve the single sing on between wordpress and moodle ?
you can contact me if you want at caste86@gmail.com in order to to avoid a fork of discussion here
Best Regards,