We have an internal application that assignes member access rights.
This is what I was thinking would be duplicated in a User Registered Event Handler. But rather than assigning access rights, it would use the rules its following to assign them to a mojoPortal role that already defines the portions of the site they can access.
It wouldn't be easy to set up or maintain a role for every combination, but I'm sure it could be done. After all, roles can not only restrict members to viewing certain pages, they can even restrict all the way down to the individual features on a page. But if you really feel you need some kind of access controls beyond what mojoPortal offers, I don't have any other recommendations.
Jamie