I would argue that with the right setup (sensors, gyros, on board computer), an automated system could do a much better job than the cyclist. My guess, is that most cyclists stay well under the 'thrown over' threshold when braking, which could be part of the reason some collisions occur (i.e. too little, too late sort of thing). An automated system could probably get you right up to that threshold, and then use gyros for stabilization (to balance the bike). The reason that this won't happen is because no one is going to install this on their bike (whereas most modern cars already have fairly sophisticated on board computers and sensor systems).

I don't think an ABS/stability system for bikes with hydraulic brakes would be unfeasible. It might even be a performance feature for DH bikes.