Try Something like this.
Have the core Jboss in ur server machine and let each user have a config on their home directories.
Check this link out.
From a security perspective, allowing multiple untrusted users to execute generic code inside a single JVM is NOT a good idea. While your symlink idea will work, each user would be able to execute any code they write with the same privs as the owner of the JBoss process. As such, I can easily drop in a war file that maps one of your server directories to a URL. If the server process owner has write permission in that directory, I can allow put and delete http commands to modify files. I can also exec anything that the server process owner can exec. You can lock it down somewhat with the conf/server.policy file. However, you still would not be able to protect users from each other. There are lots of exploits I can envision here. JBoss is not really designed to be a multi-user OS (yet anyway).