Interop

The nice thing about using MassTransit as the backend is we can interact with the backplane by publishing the appropriate message (with hub).

I can't think of a scenario you would ever publish GroupManagement<THub>. Only All<THub>, Connection<THub>, Group<THub>, and User<THub> should be used.

To publish a message from a backend service (eg. console app, topshelf):

await busControl.Publish<All<ChatHub>>(new
{
    Messages = protocols.ToProtocolDictionary("broadcastMessage", new object[] { "backend-process", "Hello" })
});

You are done!

Complex Hubs

Your ASP.NET Core might have complex Hubs, with multiple interfaces injected.

public class ProductHub : Hub
{
    public ProductHub(
        IService1 service1,
        IService2 service2,
        ICache cache,
        IMapper mapper
    )
    {
        //...
    }

    // Hub Methods...
}

Your backend service might exist in a separate project and namespace, with no knowledge of the hubs or injected services. Because MassTransit routes messages by namespace+message, I recommend to create a marker hub(s) within your backend service just for use of publishing. This saves you having to have all the hub(s) injected dependencies also within your backend service.

namespace YourNamespace.Should.Match.The.Hubs
{
    public class ProductHub : Hub
    {
        // That's it, nothing more needed.
    }
}

Protocol Dictionary

SignalR supports multiple protocols for communicating with the Hub, the "serialized message" that is sent over the backplane is translated for each protocol method supported. The Extension method .ToProtocolDictionary(...) helps facilitate this translation into the protocol for communication.