You'd probably need to build your own JMX plug-in. Adventnet (www.adventnet.com) provide an engine to create your own snmp instrumentation but I don't know the monetary cost or your client budget. They do seem to have a bunch of monitoring stuff for J2EE containers and claim to work with JBoss 3.x
You might also consider building your own JMX - using open source components - depending on the complexity required. For example, this library is available: http://edge.mcs.drexel.edu/GICL/people/sevy/snmp/snmp_package_introduction.html and I am sure there are more, if you know how to build an MBean and glue your SNMP agent code to it.
My bad. I was looking at some SNMP client code and copied the link from there. I really meant the Java wrapper for the net-snmp open source at http://netsnmpj.sourceforge.net/. There are a few issues with this so the least pain still seems to be the Adventnet way - as long as the client has the budget.
I'll have a look.
My guess (I don't know) is that they'll have a reasonable budget available since they'd be saving money not buying WLS licenses - which is their default option if their investigation into JBoss doesn't give them a warm feeling about JBoss.
if you read the changelog of the 3.2.1 version, you will see that there is a log4j snmp trap appender