Isn't the GUI just a service? Can't we just create another GUI and make MeeGo fire it up instead of the proprietary one?

I understand that harmattan UI is closed, but can't it just not be used, or the loader/core/whatever is locked in some way to prevent it from starting any other GUI? It could even lead to an UI switcher, a la android, so user can change between default MeeGo, harmattan and Cordia/hildon at will.