After lots of experimentation, I think I've finally settled on a design.  Though this whole effort has taken longer than expected, I don't think the coding will take that long since I've got experimental code that does the hard stuff.  I just need to rearrange it and clean it up.

Here is the design.  I appreciate your feedback.

Design Goals
* Support auth-server running in a WildFly domain.
* Eliminate deployment from /deployments directory to make it a proper service instead of an ordinary app.
* Eliminate need to explode or crack open auth-server.war.  Keep it intact so it doesn't need to be hacked up.
* Load user-provided overlays for keycloak-server.json, SPI jars, and theme jars.
* Allow uploading overlays from CLI.
* Allow more than one auth-server in a WildFly instance.
* Compatibility with EAP6, EAP7, WildFly8, and WildFly9.


Management Model
The Keycloak subsystem introduces a new resource called, "auth-server".  You can define more than one auth-server if you like.  The simplest form just looks like this:
<subsystem xmlns="urn:jboss:domain:keycloak:1.1">
            <auth-server name="my-auth-server-name"/>
</subsystem xmlns="urn:jboss:domain:keycloak:1.1">

On startup, this deploys my-auth-server-name.war with web context "auth".

There are two optional attributes under <auth-server>.  They are "enabled" and "web-context".  Here is an example:
<subsystem xmlns="urn:jboss:domain:keycloak:1.1">
            <auth-server name="auth-server1"/>
            <auth-server name="auth-server2">
                     <enabled>true<enabled/>
                     <web-context>auth2</web-context>
            </auth-server>
</subsystem xmlns="urn:jboss:domain:keycloak:1.1">

In a domain environment, there is an additional resource.  You assign the auth server to one or more server groups:
<subsystem xmlns="urn:jboss:domain:keycloak:1.1">
            <auth-server name="my-auth-server-name">
                     <server-group name="group1"/>
                     <server-group name="group2"/>
            </auth-server>
</subsystem xmlns="urn:jboss:domain:keycloak:1.1">

Loading the Auth Server
The auth-server.war will be loaded from the Keycloak subsystem module.   This is just a convenient place to put it.  We could actually load it from anywhere.  Note that it no longer needs to be exploded.

Using Overalys
The Keycloak subsystem can overlay or add to the auth-server.war.  Overlays do not touch the original content of auth-server.war.

To define an overaly, you just drop your files in the proper directories on the file system.  The layout is:

/overlays/<auth-server-name>/server-config/
/overlays/<auth-server-name>/account-spi/
/overlays/<auth-server-name>/login-spi/
/overlays/<auth-server-name>/other-spi/

/server-config optionally contains a single json file that replaces keycloak-server.json
/account-spi optionally contains a single jar file that replaces keycloak-account-freemarker.jar.
/login-spi optionally contains a single jar file that replaces keycloak-login-freemarker.jar
/other-spi optionally contains one or more spi jar files to be added to WEB-INF/lib.  Theme jars also go here.

Location of SPI jars and other user-defined overalys
For now, I'm planning to have the /overlays directory in the Keycloak subsystem module.  They could, for instance, go in a /keycloak directory such as <wildfly-home>/keycloak.   Any thoughts on this?

Uploading overlays from CLI
It is already possible to create overalys via CLI.  You upload the content and assign it to deployments and server-groups.  The CLI commands for this are rather complicated and you really need to know what you are doing. 

We could make this easier by adding simpler CLI operations to the Keycloak subsystem.  However, I think we should hold off on this until we find out if the directory-based approach is acceptable to users.  In the mean time, we can just document the CLI commands needed to upload overlays.