Presently we can only poll StatusManager for errors. It would be nice to be able to subscribe to these messages.

One idea was to allow custom StatusManager implementations.
This sounds like a nice and sane idea in its own right.

However I feel that providing subscription to error events and allowing users to provide alternative StatusManager implementations are orthogonal concerns.

In fact I feel that application should be able to subscribe to logback errors no matter what StatusManager is used stock or custom.
In other words I suggest it to be part of StatusManager interface/contract to provide subscriptions.

Which brings me back to the idea that it would be best to provide subscription (listener) facility in the standards StatusManager.

On yet another account http://jira.qos.ch/browse/LBCLASSIC-58 may be enough for most practical users.
And I suspect that the most common LogbackErrorListener implementation will simply log to System.out.
In any case we badly need to get these errors out of Logback.

BTW an alternative implemenation of LogbackListener might log to a file.