Is the application crasshing but jboss staying up? I have found that the sessinos will only move over if the jboss instance is shut down normally, or the java process crashes suddenly. If the app simply fails, or gets undeployed it most likely wont fail over.
If your architecture requires that each apache only handles a single cluster, then whether there is any solution depends on your H/W loadbalancer; e.g. if it could detect the 503 and trigger a failover to another apache, that could work.
If all apache servers can handle all clusters, then the issue can be handled at the apache layer by using the mod_jk's "domain" feature: