I would question the effectiveness of moving the sensor, rather than a lens element, to provide image stabilisation. ...
Good points though in-lens IS involves moving one lens group, and a consequent degradation of the image quality. Few people seem to complain about that thoug it is sometimes mentioned. I doubt that a few mm displacement of the sensor would be any worse.
It might also be the case that KM are clever and make sure that the sensor is centrally aligned at the exact moment that the exposure is taken. (Using the short interval between the shutter trigger being pressed, and the system firing the shutter.) Obviously for a slow exposure travel would be greater, so they would start off-centre, and try and make sure that the offsets at the start and end of the exposure were equal and opposite. Thinking about this, it seems to me that during the exposure a given part of the object passes in front of the lens, and the distortion and aberrations will change depending on the part of the lens being used. So although large motions might seem feasible with an APS sensor and a FF lens, in practice large motions will lead to image degradation due to using different parts of the lens.
It also seems to me that if you implement AS on an APS DSLR with an APS only lens, you have little room to move the sensor due to the reduced image circle. So, FF will bring no new challenges, apart from the greater mass, and associated costs, as mentioned above by someone else.
Leif