Thanks, that helped. In case someone else comes across this thread looking to do the same thing in the future, I ended up making two changes in addition to the one in my initial message. First, I edited conf/log4j.xml and added:

<appender name="DEBUGCONSOLE" class="org.apache.log4j.ConsoleAppender">
 <errorHandler class="org.jboss.logging.util.OnlyOnceErrorHandler"/>
 <param name="Target" value="System.out"/>
 <param name="Threshold" value="DEBUG"/>

 <layout class="org.apache.log4j.PatternLayout">
 <!-- The default pattern: Date Priority [Category] Message\n -->
 <param name="ConversionPattern" value="%d{ABSOLUTE} %-5p [%c{1}] %m%n"/
>
 </layout>
 </appender>

 <category name="org.jboss.web">
 <priority value="DEBUG"/>
 <appender-ref ref="DEBUGCONSOLE" />
 </category>

That causes Tomcat's debug messages to be printed to the console without doing the same for all the other modules.

Next, I extracted the file org/jboss/web/tomcat/tc5/session/LocalStrings.properties from tomcat50-service.jar and moved it into org/jboss/web/tomcat/session (under the deploy/jbossweb-tomcat50.sar directory). That prevents the error message I quoted, and allows Tomcat to display a few additional debug messages.

Hope that's helpful to someone. Thanks, janilsal, for the pointer.