Well, I suppose that's an option, but then you are back to having to disallow surrender. Otherwise an empire on the way out could simply surrender to someone they want to have the immunity. yeech.
And then there is the matter of an empire with two planets left. One guy kills off one planet and the other guy kills off the other planet. SO you give the immunity to whom? The guy with the higher player order? Double Yeech.
Not trying to poke holes in your idea, I do like it on principle. It's just I don't see a way to execute it that doesn't end up with me having to mediate a lot of hissy fits.
Geoschmo