Ok, so as an update.. I've located a bundled zip file with what appears to be the web engine. The name of the .zip file is jbossweb-src.zip and is packaged at jboss-4.2.1.GA-src\thirdparty\jboss\web\lib.
Now to work on getting the changes to compile.
It would appear the jbossweb-src.zip, even without changes, cannot be compiled, jarred, and included with the JBoss build. It won't deploy.
When I have only a few classes that need to be changed, I usually compile those sources against the original jar file, and then replace only those changed classes in the original jar file.