I assume you are referring to Windows Workflow here? Our Docking/MDI product has document windows that are effectively the same as a normal WPF ContentControl in terms of presenting content, just that their chrome is styled to look like tabbed MDI and have the docking functionality.
When setting child content for the docking window, you simply set the DocumentWindow.Content property to any FrameworkElement (or Control). Then the ContentPresenter within the docking window's template renders the content. It's not really possible for us to do anything asynchronously related to that.
For large complex controls, it's generally up to them to virtualize their contents when rendered.
Some controls might also take a while to load their data. For those kinds of things, you would want to load them separately on your own, perhaps in a dispatched call, and then once that's complete, set the resulting FrameworkElement as the DocumentWindow.Content.