* No. Whitebalance is a property of the whole picture, not of separated pixels/sensels.
* if you're talking about background and frontscene in a 2d picture, you talk about a depthmap or simpler masking.
No.
* could be able to save two measured whitebalance-values, but i assume, there's no tif-tag for a second metadata like this, neither in cr2 nor in dng.
* Finally you would do the job manually, as it was before..
regards chmee