Lars,
The only way I can think of to do this would be to write a custom handler. While it's not very well documented at the moment, you can create custom java.util.logging.Handler's. You could write a wrapper in a sense around around the logback's configuration. I think they have a BasicConfigurator or something like that.
You register a custom handler like so:
<custom-handler name="logbackHandler" class="org.jboss.LogbackHandler" module="org.jboss.logback">
<level name="DEBUG"/>
<properties>
<property name="nameOfASetterMethod" value="the value to set" />
</properties>
</custom-handler>
<root-logger>
<level name="INFO"/>
<handlers>
<handler name="CONSOLE"/>
<handler name="FILE"/>
<handler name="logbackHandler"/>
</handlers>
</root-logger>
That said there is probably no real need to do that. The application server logger will log the messages even if you are logging through a different façade. You can set-up different file handlers if you want to write to your own files.
I realize logging in JBoss AS7 could really use some better documentation. I do plan on updating that when I find the time :-) And really I just need to make the time.