Async Web client integrating with Windows Workflow Foundation (WF4)

Question

I am designing a workflow using WF4 that is supposed to be consumed by a Web client. The workflow aggregates WCF services and is triggered by a Receive activity that is in turn spawned from a Web page. While the workflow is being executed, the user's Web session is active, and certain workflow activities may need to inform a user, i.e. Web page needs a feedback about certain workflow stages.

I would like to stick to WCF-based activities (Send, Receive etc.), but not quite certain how to implement feedback to a Web page. Let's say this is the scenario:

1. User clicks "Start" button on a Web page to start a workflow.
2. Workflow service creates a workflow instance and responds with a instance Id.
3. Next, "DoSomething" activity is executed on a server
4. When DoSomething is completed, user should be presented a new Web page. 5-NNN. Other activities are executed, some of them also requiring feedback.

Steps 1-2 can be implemented using ReceiveAndSendReply compound activity that consists of Receive and SendReply. Step 3 is some custom Code activity that completes shortly My first thought about step 4 is that it can be implemented using Send activity, but then Web application should act as a WCF service and provide an endpoint which does not fit a Web client. How else this feedback can be implemented?

Thanks in advance