Twitter is running rails on most of their front end. Fauna’s client is actually built and released by twitter, so you can be pretty certain that it’s up to date and stable on large workloads. Looking at the history of commits shows that there are frequent improvements being pushed to it, which is great.
Most likely Authlogic would need to be customized to work properly with Cassandra. In particular, it appears to provide certain methods based on named_scope and relational data.
It does appear that someone has built a plugin for DataMapper support in Authlogic: http://twitter.com/collintmiller/statuses/2064046718. You may be able to use that as a starting point for making it compatible with Cassandra.
Good luck!