Never used "Gnome Shell"... I still use MM (Ubuntu 10.10)... so I don't know the "structure" of the themes for it... I just know that "Gnome Shell" use Mutter as compositing window manager, but I didn't "study" it.
Doing themes for Metacity is less "graphic" than Emerald... there's code... and considering this theme and the layout of Metacity, I think it's not possible porting it to Metacity... but I've done just 2-3 (simple) themes for Metacity, so maybe I'm wrong.