Miguel demoed a Xamarin.Forms implementation using it, which is kind of the other way around.
Must be... in fact, it may be a great idea! This API must be simpler, so it must be much more doable than the other way around.
I think I'll take a look at it, my guess is the layouts might be tricky to translate but with some sensible hard coding perhaps not too bad. Sadly the state of C#'s Qt and GTK libraries weren't great last I looked.
Gtk# is... OK. I've used it a little bit and for the most part it implements everything the C/C++ version does.