That takes care of the first 20%... but what about the cheap AC->DC transformers that sit between your house wiring and your devices? I'd love to be able to switch each outlet I have between 110VAC/15a, 12VDC/3-5a and 9VDC/500Ma-2a, and do away with wall warts altogether.
Since you have different DC voltages you either need DC wall warts (not achieving anything), per outlet transformers (sounds expensive and turn the socket into a wall wart) or one circuit per outlet (sounds expensive and needs big conduits). And because of fire risk, property damage and whatnot you'll never get to use the same plug, tripling each outlet. Then you need something fancy to flip each socket at the source and what part of the socket is powered at the sink in a safe way, for every outlet. And I doubt 9VDC/500mW-2A matters much economically, if you want the aesthetics embed the wall wart in the wall, it'll probably be less hassle than the alternatives.