I agree with Toby, if you don't need this additional attribute to be on AuthLogic's UserSession object, then it may be simpler to store the value in the session hash itself.
But in my specific case, and perhaps in yours too, I have information that I want on the UserSession record specifically rather than the session hash, because I want to access it in a callback in the UserSession model, where "session" is unavailable.
Here is an older blog post describing how to store an additional attribute on UserSession:
http://railsblog.kieser.net/2010/03/authlogic-custom-logins-and-persisting.html
EDIT: I didn't have much luck with this approach myself. To do the things that required information from the session hash, I put that logic in the controller instead of the model.
AuthLogic actually has certain callbacks that execute controller methods. For instance, AuthLogic will call last_request_update_allowed? if your controller responds to that, right before setting last_request_at.