That's possible to implement on 5D3 (and hopefully all the other dual-slot models which will run ML), although it's something that has to be decided before taking the picture or recording the video. For example, auto-switching to CF when going to movie mode and back to SD when going to photo mode should be doable, but that means photos taken in movie mode will go to CF.
Burst mode may be tricky - changing settings during a burst sequence doesn't always work, and the behavior is model-specific (for example, 500D allows changing exposure during a burst, but most newer models refuse such changes). So, it's probably tricky (not impossible, but not straightforward either) to switch the card during a burst sequence (if ML detects you are taking more than one picture).
I don't see this feature as a priority, but if you have some time to spare, feel free to give it a try. There is already a setting in ML that switches the card at camera startup, so you may use it as a starting point.
