MJPEG movie recording has many benefits over h.264 recording, it places less demands on the CPU when compressing, which means less energy used, less heat generated & longer battery life.
Powershots use dedicated hardware for encoding, there's no way the main CPU could handle HD h.264. It's not a given that MJPEG support still exists in the new cameras, and if it does, it's not a given that it would save power.
Since MJPEG is much less space efficient, it is likely that that the camera doesn't have the IO bandwidth to save HD video in MJPEG at all.
Re-enabling MJPEG would depend on the old code still existing in the canon firmware.
Perhaps even a JPEG image sequence could be used instead? - this would get around the 4GB limit, (as each video is just a sequence of images).
This would be incredibly inefficient, and even less likely to work than regular mjpeg. The canon firmware would probably fall over due to having too many files.
