I've sent a PR for this:
https://github.com/keycloak/keycloak/pull/811

It's a pretty big change in the way the Auth Server is started when the KeyCloak subsystem is used.  The WAR is no longer dropped into the standalone/deployments directory.   This is especially helpful for domain deployments, but it makes standalone cleaner as well.  It will also be important for Feature Pack installation.

The main difference you will see right away with this PR is that the appliance dist now uses the subsystem to launch the Auth Server.

Here are some notes about how everything turned out.  Next, I'll update the documentation if there is no major rework that needs to be done after the PR is reviewed.

• The WAR for the auth server now lives in modules/.../keycloak-wildfly-subsystem/main/auth-server.  By default, it is unexploded.  If you want it to be exploded you can unzip it into that same directory and set the "auth-server-exploded" property in module.xml.
• A new Auth Server is declared in standalone.xml/domain.xml.  You can have more than one Auth Server in the same WildFly instance. 
• <subsystem xmlns="urn:jboss:domain:keycloak:1.0">
              <auth-server name="main-auth-server">
                  <enabled>true</enabled>
                  <web-context>auth</web-context>
              </auth-server>
  
  
• The "enabled" attribute can be toggled at runtime to make the auth server undeploy/redeploy.
• If you have more than one auth-server, the web-context must be unique.
• In a domain environment, all specified Auth Server deployments are propagated to all servers using that profile.  The same is true for overlays uploaded through the new CLI operations.
  
• There are two new CLI operations that act on an auth-server.  They are "add-provider" and "update-server-config".  Currently, you can only execute these operations in the latest version of CLI GUI.   We should discuss if we need to add support in plain CLI.  The long term goal would be to add this functionality to the Keycloak Admin Console.
• "add-provider" adds a provider jar to an auth-server
• "update-server-config" overlays the keycloak-server.json for an auth-server.
• If a keycloak-server.json file is found in standalone/configuration directory, all auth-server instances will still use it regardless of any update-server-config operations.
  
• EAP6 does not yet support all this.  We should discuss whether or not this functionality should be backported.