Atlassian Crowd supports authenticating through remote directories, using LDAP, OpenID, Google Apps, Active Directory, etc. It also supports Custom Directories, for creating your own authenticator. Is it possible to use this interface to implement a CAS client that can authenticate towards an external CAS server?
I don't know much or anything about Atlassian Crowd, but it seems like they have a connector available for Spring Security, and Spring Security does already have CAS support. So you might be able to use what they have and what Spring Security has to connect it to your CAS instance. See https://confluence.atlassian.com/display/CROWD/Integrating+Crowd+with+Spring+Security