General help

Perhaps it would be better to add a custom field new account form, where new users register. It possible to add a field there?

One option is to create a html document of the license agreement and upload it where the public has access. Insert the URL of the document in the Site policy URL field (Site administration -> Security -> Site Policies). That way a "I understand and agree" checkbox will automatically appear on the email-based self-enrolment form.

I don't think you can do that by simply configuring the self enrol plugin.

If this is very important to you and you are ok with writing code for Moodle, and maintaining it at upgrades, you can try creating an enrolment plugin which extends the self plugin. I have never extended an existing enrol plugin. BTW I recommend not modifying the core code directly unless it absolutely can't be avoided.

Or you can do as suggested and add the condition at registration.

Or you could add some kind of course activity in each course, like a choice type activity containing a description and a link to the policy and make other resources available only if that one completed.

http://docs.moodle.org/23/en/Choice_module

But the official Moodle way for agreeing to policies is at registration.

I'm curious to hear if there are other ideas.

A specific course-license module would be interesting, with different kinds of licenses to chose from for each course. Maybe it exists.

I'm curious about other suggestions that come up.

Good luck.