I am currently developing the same kind of application, and I have to deal with the same problem due to firewall restrictions. What I was doing, till recently, is that I used some XML-formatted files to implement client/server communications via a servlet. But I found that JBoss now includes a module named JBoss.net which allows one to create "web services".
I believe you should be able to have the jnp server listening on port 80 & have the client requesting onto that port too.
It should (only) be a matter of modifying your deployment descriptors. As long as you can have non-HTTP traffic going tru the firewall (sockets open tru' 80 are not HTTP request per se).
You will be probably facing some other issues.