So-called "Carbon Emacs" is GNU Emacs on Mac OS X with native graphics libraries. I prefer it to X window GUI. It has been once supported in emacs-cvs during 21.3.50 through 22.0.97. The support ended when an ebuild was replaced with one from the emacs overlay. I don't know whether it was intensional or not. See #64362 for the old support request and fix. A patch will follow.
Created attachment 148113 [details, diff] a patch against emacs-22.2.ebuild in the prefix tree The name of option is called "aqua" after the old bug discussion. But I change the policy of X/aqua choice so that user can choose only an option. (Old ebuild let user to set USE="X aqua" and aqua has precedence.)
thanks, I applied your patch with some small changes.