I'll field this one.
If you have a static module which you want considered as a URL stream handler provider for a standalone AS server instance, you must add your custom module to the org.jboss.as.standalone module import list like this:
<dependencies>
<module name="javax.api"/>
<module name="org.jboss.logmanager" services="import"/>
<module name="org.jboss.as.jmx" services="import"/>
<module name="org.jboss.as.server" export="true"/>
<module name="jcifs" services="import"/>
</dependencies>
This should cause your URL handler to be recognized.