This part 4 of my 6 part series on the EditingContext.
In addition to having a host provide an instance of a type to be used within the designer, it can also be used to pass an instance that will route callbacks to the host. I covered this briefly in a previous post (Displaying Validation Errors in a Rehosted WF4 Designer). In that case, we provide an implementation of IValidationErrorService, which the designer infrastructure will call, if available, towards the end of a completion of a validation episode. In the sample application in that post, we use that instance to route, and display the validation errors in the system.
Rather than duplicate the code, I will simply encourage you to check out that post and think about the way you could publish a service that your activity designers know about, and use it as a mechanism for calling methods within the hosting application.