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I've discovered how to do this:-

Add references to the relevant assemblies to the Workflow designer 

In TypeProviderService.cs

in method GetTypeProvider add the following:-

   typeProvider.AddAssembly(typeof(System.Workflow.Activities.ReceiveActivity).Assembly);

and in Workflowdsidetabservice.cs

in method AddViewContent add the following:-

    LoadSideTabItemsFromAssembly(typeof(System.Workflow.Activities.ReceiveActivity).Assembly.GetName(), standardSideTab);