You may able to do this dynamic by getting appenders and modifying through runtime API. If log4j.properties file going to change manually, you can turn-on watch feature (Im not sure what is that called) to update the logger automatic.
If none of this going to work!! You can always write custom wrapper to-do what you want.. That�s the prettiness of open source.
[ October 05, 2004: Message edited by: Bharathi Balasubramanian ]