I don't think that's going to work, because the ScriptManager needs the Http Context to work; internally, it is using that to access the request/response lifecycle. Since the dependency can change OUTSIDE of that lifecycle, the ScriptManager won't be able to be instantiated because it only works while IN the lifecycle. That is the problem with statics and stateless ASP.NET.
The only real option is to consider a polling mechanism on the client side, or check out HTML 5 server side events with ASP.NET, or even HTML 5 web sockets.