7.1.1 uses JBossWeb 7.0.13 so perhaps it's not that out of date. Don't know about your issue but I recall valves (another Tomcat construct) being defined in jboss-web.xml so perhaps that is a place to start looking...
Have you had a look at https://docs.jboss.org/author/display/AS71/AS7+Cluster+Howto?
While this does not specifically allow you to configure session replication using the memcached session manager (AFAIK), you may find a caching strategy using Infinispan with similar characteristics and behaviour.
At the present moment, you can't swap out the entire org.apache.catalina.Manager implemention via configuration alone. You can, however, swap the distributed storage mechanism (you'll need to adapt the memcached session manager to another SPI first) used by AS7's distributed session manager implementation.
To do this, you would:
1. create an implementation of the org.jboss.as.clustering.web.DistributedCacheManagerFactory interface (and all dependent interfaces)
2. create a module that defines a META-INF/services/org.jboss.as.clustering.web.DistributedCacheManagerFactory file that points to your implementation
3. replace the default implementation (org.jboss.as.clustering.web.infinispan) in $JBOSS_HOME/modules/org/jboss/as/clustering/web/spi/main/module.xml with your custom module.