GNOME developers definitely accept patches from outside contributors.
I believe that whatever muffin does could be done in mutter. Even Cinnamon could probably be implemented in extensions and I think GNOME Shell developers would be interested in accepting a few patches to make that possible (if it isn't already).
But the Linux Mint devs would rather keep their GNOME 3.2 forks of GNOME Shell, Mutter, and Nautilus.