Plugins fire based on some data action occuring in CRM via the CRM platform. So, you need a way to invoke such an action. One approach is to create a dummy entity (that is not used for anything else), and register the plugin against one of the messages
(e.g. Create or Retrieve) of this entity. You can then attach a javascript function that then causes the event (Create or Retrieve) - the easiest way to do this is probably to use the REST endpoint - see
http://msdn.microsoft.com/en-us/library/gg334427.aspx