I've looked at the JBossWeb source code and I believe that what you are seeing may be down to content type. The vague section you quoted gives some indication of the problem.
maxPostSize only appears to apply if the request method is POST and the request content type is "application/x-www-form-urlencoded". Even then, if maxPostSize is exceeded, you simply see a log message and the request is not rejected. When maxPostSize is exceeded in this way, it's only the automatic form parameter parsing that is skipped. Any code that later attempts to read the body of the request can do so (the whole thing) without a problem.
I'm interested to know whether you found a solution back in April 2009. Having investigated the limitations of the JBossWeb config parameters, we're considering using Apache to apply these low-level HTTP limits.