Hi Jon, thank you for your input. Highly appreciated!
I believe you can set up self enrolment to only allow users within a certain cohort til use the self enrolment method. In this case you can specify which role they should have upon entering the course, which is what I am looking for. The possibility exits already. BUT - I also need students to be able to self enrol. And you can't - it seems - have two instances on self enrolment set up unless you do it on course level/for every single course, which is not an option for us due to sheer volume.
I see what you're saying about cloning the existing self-enrolment plugin and editing the database. I'll need to think about that