[logback-user] Is there a way to define a variable by making a call to Joran?

I looked quickly through the class stack for the Joran configurator, but I didn't spot anything. Is there something I'm missing? I want to have an XML file with some variable references, configure explicitly with Joran, and set some vars. I can write code to edit the XML or to set system props, but I was hoping for something cleaner.