Using the plugin generator is generally a great way to get started and see how a plugin should be structured. I recommend it.
Now, regarding how to get ruby code into the front end: those are two different applications. The front end application (Ember) will have to request it from the server (Rails) somehow. Normally we do this via an AJAX call, but you could also do something like add to the SiteSerializer so that it’s sent automatically as part of the forum data.
Otherwise you will have to look up how to add a route/controller in rails and send the messages as JSON.