OK , the issue is with the file cluster-examples-service.xml not being well formed. If you open it in a xml editor you'll notice that comment tags have space between them. Fixed all those and now HASingletonMBeanExample is deploying fine and I see jboss.examples domain in jmx-console :)
You found it before I could reply. :)
The intent is that those beans are commented out, as we don't want to deploy example services in a real server. Whoever did the commenting out did so in a highly obscure and easy to miss fashion. Makes it easy to uncomment the beans, but only if you pay close attention and see the pattern.
TBH, I'm not sure why that file is there at all (since we don't want to deploy example services in a real server.) It was there before I started at JBoss and its never come up on my priority list to see if there's a reason not to include it.