How do I use logback Pattern in custom Layout?

I am relatively new to Logback. I have figured out how to create a custom layout.

But sometimes based on information in the event I would like the ability to use a pattern in the doLayout method. Is there code for converting an Event using an Pattern broken out into a separate reuseable classes. Something like this:

public class MyCustomLayout extends LayoutBase<ILoggingEvent> {
     String doLayout(Event e) {
         [Based on some condition]
         Pattern p = "%mdc";
         String convertedPatternToString = p.process(e);
         ....
     }
}

Is there a Pattern object that will do that in logback?

I realize I could do this with two separate appenders (one with PatternLayoutEncoder and one with MyCustomLayout.) But I really have needs to have it one layout class.

Thanks.