1) Host app uses logback, loads JoranConfigurator and configures logback
2) Host app loads plugin jar with a plugin classloader
3) Plugin loads its own logback.xml and would like to extend existing LoggingContext with plugin logging configuration and to use custom appender specified in pluging logback.xml
4) JoranConfigurator fails, as it fails to load plugin's appender.

There should be a way to specify classloader for JoranConfigurator or for individual configure calls. Configurator then should use specified classloader to load classes referenced in configuration file.