Alright, got it figured out. The native management port (9999) was removed in the Final version of Wildfly 8. Now there's only 1 management port (9990) and it has multiple previous ports multiplexed over it, including the JMX. It was still available in the release candidate of Wildfly 8, hence all the confusion about the online instructions and configs available elsewhere online.
So the key is to specify the proper protocol, which is not the remoting-jmx now, but http-remoting-jmx. The URL to connect to the server must be like this:
service:jmx:http-remoting-jmx://<server_host_or_ip>:9990
(this is versus service:jmx:remoting-jmx://:9999 in previous jboss/wildfly servers)
Finally, no need to mess with standalone.xml config. All config tweaks to make it work on Jboss 7.x won't work for it. It all works out of the box with proper protocol and port number. Just make sure to create a jboss user in ManagementRealm.