I was doing an initial investigation of the new AS back then. We have quite a lot of apps and services going into JBoss 5.1.0 at the moment, and only now actually migrating that to 7.
This aspect will no doubt be looked at proper, but my own personal feeling is that we would try to port the Appender code ourselves and not necessarily have it triggering JMX events, but rather direct to the filtering and forwarding logic.
We used the JBoss Shutdown notifications to pre-shutdown some service elements. With migrating from MBeans to OSGI Services, it could transpire that when that code migrates, the OSGI stop life cycle processing will give sufficient time to achieve that without having any other hooks into the container.
Sorry that sounds vague. It's all I know at this time.