I am working on a custom session manager for Tomcat and I got it working with one caveat, I have to put the jar(and all the associated jars) in the CATALINA_HOME/lib directo
As of Tomcat 7.0.27, the META-INF/context.xml file is processed by the commons-digester and the effective ClassLoader is set to that which loaded the ContextConfig class -- one which firmly resides in Tomcat's server ClassLoader, which is outside of the ClassLoading chain of anything with access to the webapp's classes.
So, without modifying the Tomcat source, there will be no way to load your Manager from within your webapp. Consider joining the Tomcat users' mailing list to discuss such a feature.
EDIT 2015-067-01
If you want to specify a ClassLoader for your web application, you'll need to use the element within your : http://tomcat.apache.org/tomcat-8.0-doc/config/loader.html