I really need a reliable way to turn on debugging from a system property. I don't care what you do internally when you see {{logback.debug=true}}; why not adding a useful listener when the property is set to {{true}}?

That way, users of logback wouldn't have to know how the debugging works (so you'd actually cut a dependency instead of introducing a new one).

Internally (and for more complex scenarios), you can offer the listener API.