It is possible to use stock upstream GNOME technologies with minimal changes to Unity (but you'll lose things like indicators.)
There are patches that improve the Unity experience, however they are not a direct requirement for running Unity. There is a GIO patch (submitted upstream), a GTK patch for appmenu support (which are not upstream), so you would lose the global menu.
Most of the patches in Unity today are for mutter/clutter, but porting Unity to compiz will remove the need for these. We are working very closely with Compiz upstream (Canonical has hired one of them) to basically make Unity a compiz plugin.