I think I've figured this out. I'm not sure if it is a bug or a feature? It would appear that calling deploy on the main deployer with a relative url to a war file is processed by the Tomcat deployer and calling deploy with an absolute url to a war file is processed by the JBossWS deployer.
I'm not sure if this is intentional or not. However, it is I can now deploy a nested war simply by providing the main deployer with an absolute url to my war file.