This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
JBoss is instrumented through JMX, so in theory you should be able to get an Mbean, prove to it you have the necessary admin credentials, and then tell it to change the log level. However, that's about as much as I can tell you. Whenever I need to change the log level, I just make the change in jboss-log4j.xml in the server/default/config directory.