There have been quite a few examples circulating on the web on how one would use SignalR together with Web API. It all started after Brad Wilson had a great example on calling SignalR from API controllers at NDC Oslo 2012.
Even on this blog we looked at calling SignalR from Web API ITraceWriter to provide realtime tracing capabilities.
How about, to avoid any controller-level noise, messaging the connected SignalR subscribers from ActionFilters? While this approach might not be applicable in all scenarios, when it is, I think it could provide a nice layer of separation.