Looks like you’ve already got a solution, but in case it helps anybody else: I wrote a MonoGame module for my Gemini framework, which shows one method for doing interop between WPF and MonoGame (mostly based on code from other people).
The end result looks quite similar to the XNA module:
http://documentup.com/tgjones/gemini#what-modules-are-built-in/xna-module
https://github.com/tgjones/gemini/tree/master/src/Gemini.Modules.MonoGame (in Controls/DrawingSurface.cs and Services/GraphicsDeviceService.cs)