Adding Users to a User Group Based on a Custom Question

It would be helpful if there was a feature that allowed site managers to use a custom question in the user registration to automatically add people to a particular user group.  In the case of my university, we'd like to add users associated with different colleges on campus to user groups for those colleges and there isn't anything distinctive about their e-mail addresses, so we can't use the "allowed domains" feature.  

I know that we can use filters to batch add folks to user groups, but I think something that is automated would help ensure more timely user group updates.