Hmmmm yeah… this is a little less fun but you could keep all the “callback functions” on a service and have a class-based helper that could reach into the service for an arbitrary callback function to use 
Or maybe you could use something like the ember-composable-helpers approach like the map helper? EDIT: though i guess that gets you back to the same problem of “where does the action live” so I guess a service is the only real answer I can think of.