I have noticed that the TraceAppender already does something similar to what you suggested, so I did something similar.
I added a Category property to the DebugAppender of type LayoutSkeleton, it defaults to new PatternLayout("%logger")
I created a LayoutClass named ReturnsNullLayout that derives from LayoutSkeleton, in which all methods are NOOP. In LayoutSkeleton I made the Format(LoggingEvent) method virtual to be able to override it in the ReturnsNullLayout class and avoid creating a StringWriter instance for no reason.
I wrote tests for the DebugAppender, none existed. I copied the TraceAppender tests. I also wrote a test for the ReturnsNullLayout class. I ran the tests and they all passed.
My patch also adds the files to the vs2012/2010 and 2008 solutions. I can test that 2012 and 2010 build, but I dont have 2008 to test if it builds. I also modified the .NET core project files which I am not familiar with, but it seems to work in visual studio.
Let me know what you think.