Thanks everyone.
Hashes won't work because the info needs to be readable to support the enrolment; hashes are 1-way.
We already have a separate enrolment plugin with it's own db, so I'll look into that; the issue is primarily UI. The usual tradeoffs apply; you can have it quick, you can have it good, you can have it cheap... pick 2. Quick and cheap win out, and the proper fix is delayed till later, after the fan gets all dirty.
It looks like from Moodle 3 it will be possible to extend the course settings navigation, that looks like what we're after, and we're upgrading later in the year. So maybe quick fix for now (after testing) on our existing setup, and do it properly as part of our upgrade process (which is always quite involved because of the plugins customisation we've done).
Cheers,