That's what I thought originally, but this would require the double amount of data storage space on a Blu-ray disc. It's not just to have black covering bars left and right of the 4:3 image but would require an extensive amount of reframing, horizontally and vertically.
James Cameron provided a film of the full Super35 camera negative from Terminator 2
and there you could see how the 2.35:1 and 4:3 extraction frames constantly moved.
According to my technical knowledge you can't do that on Blu-ray, yet.