I think it's better to address this by disabling JSF managed beans when 299 is present, rather than by creating extra rules about how the two interact. Any reason to take the latter approach?
My only concern here is that someone writes an application that they have been deploying to Tomcat or Jetty and then they turn around and deploy it to JBoss AS for production (or pre-production). All of a sudden, the playing field changes. This seems like a very realistic scenario, especially for people migrating.
-Dan