I suspect they're engineering a similar mode for MP3 -- hopefully something a little nicer than Joint Stereo, which basically works by doing a mono mix and specifying which frequencies are louder in which channel. No, this doesn't work very well. Concievably, we could see something like VBR on a per-channel basis, but I suspect this would cause existing decoders to collapse. I do believe it's possible to place extra data between MP3 granules; I suppose they'll get their backwards compatible surround mode worked into there.
That is precisely how MP3 mid side stereo mode works. It takes the sum of the channels (the common sounds) and encodes with a higher bitrate than the sounds that differ. Joint stereo is a mode where the encoder decides whether to use Mid-Side or true stereo for each frame depending on the stereo seperation. Joint stereo gives better results than true stereo at the same bitrate.
The mode you describe (mono with frequency info) is Intensity Stereo which few encoders even support.
