The Layout chapter indicates it supports property value retrieval via %property{key} and states that the logging context is where one might put a property (along w/ System properties). I believe log4j did this w/ the <param name="key_name" value="some_value"> element in the <appender> and referenced it via %properties{key_name}.
I'm not seeing how to do this in Logback in XML. Would it go in the <logger> element? What's the syntax? Could someone please point me to the relevant section in the appropriate chapter of the manual?