Server-To-Client RPC with Response in ASP.NET Core SignalR

When using ASP.NET Core SignalR, we can perform invocations which don't return anything, invocations which return a result, and invocations which results in a stream of results. Unfortunately, invocations which return a result or a stream of results are available only when invoking server from a client. If a server wants to invoke a client, the only option is invocation which doesn't return anything. ASP.NET Core SignalR is supposed to bring streaming from client to server but again only as part of an invocation from client to a server. Sometimes there are scenarios where we would like a client to be able to respond to invocation - a server-to-client remote procedure call (RPC) with a result.

What We Want

ASP.NET Core has a concept of strongly typed hubs which allows for representing client methods as an interface. For a server-to-client RPC with a result scenario, such an interface should look like below.

public interface IRpc
{
    Task<MethodResponse> MethodCall(MethodParams methodParams);
}

With the following, corresponding, strongly typed hub.

public class RpcHub : Hub<IRpc>
{
    ...
}

The hub doesn't need to have any methods. Of course, we want to handle connection events to maintain a list of currently connected users, but if a trigger is something from our infrastructure (message from a queue, change from a database, etc.) that's all we need. Let's assume that's exactly the case, that clients methods are being invoked from a BackgroundService which is listening for that trigger. Bel