This question is more like a pull request..
We use CAS with all our services and therefore wrote a module for question2answer: https://github.com/alysisLeonis/qa-external-cas
As Q2A is a cool project and we thought it was missing a pure CAS (besides qa-external-casldap of brenard which uses ldap as well), so we extracted it an made if confable.
How could that be integrated into Q2A ?
I would be happy if a maintainer or plugin developer could merge/pull that or help us making a plugin, I will not yet make a pull request, as it is not the Core neither a plugin..