I'm actually integrating eyeOS with the phpBB user manager, so users who have an account in the forum of my site (which are also used for other things in the site) can access to a eyeOS account.
How it is working:
-- A user must firstly register on the forum.
-- Then if the registered user wants to use also eyeOS, is just login in eyeOS with the forum user+pass and then eyeOS creates the user (after checking the db) and asks for a new pass (only for eyeOS).
-- After all, is just login in eyeOS with the pass created, it don't checks the db anymore for that user
