Online Total in forum_list via plugin

I'm wishing to expose only the {xen:number $onlineTotals.members} so that I can access it in the forum_list template.

I can make it work by hacking XenForo_ControllerPublic_Index and duplicating the required code from XenForo_ControllerPublic_Online but I'm a little lost on how to do this properly with a plugin.

After setting up an event listener do I simply create a new actionIndex() function that would have the bits of code I need or do I need to duplicate more. I saw the link for adding new actions ( ) but I'm curious what you need to do for already existing actions.

On a side note is there a detailed discussion of the inner workings of the Plugin system? I see myself doing more plugins and having a better grasp on the technology would probably be a good time.
I guess I simply want to know how the plugin system works with event listeners when trying to add something to an action that is already in XenForo