Severity metadata is supported for Model MBeans, so if your notification source is a model mbean you can describe its notifications and their severity.
IIRC, this doesn't mean that the severity is attached as part of the notification though. The recipient of the notification would have to connect to the MBean server and query the notifications metadata about the severity level.
Of course, your implementation may add any severity value as part of the notification (whether from the MBean's own metadata or from somewhere else). The notification supports an extra UserData field for this purpose (notice that the user data should be serializable though).
I figured this would be the approach. However, while JBoss group is busy re-implementing the monitoring scheme for JBoss 3.2.4, perhaps it would be a worthy inclusion.
For example, AlertListeners could subscribe to a severity level notification.