Calling SignalR hub clients from elsewhere in system

This is the correct way for SignalR 2.x:

var context = GlobalHost.ConnectionManager.GetHubContext<MyHub>();
context.Clients.All.addMessage(message);

Basically, you can use the dependency resolver for the current host to resolve the IConnectionManager interface which allows you to get ahold of the context object for a hub.

Further information can be found in the official documentation.