They should go where the install script puts them, bizarrely enough ;-)
A deployer is a new concept in JBoss 5, it replaces the server/default/lib dir, it's shared betweeen all apps.
And there might be support for using custom Web Bean jars included with the WAR at some point(?)
Probably not in an AS that embeds Web Beans, no. You can always do this in a servlet container like Tomcat.