What I mean is the question if the Model S provides an emergency and redundant system for the brakes, the Model S can easily detect if the hydraulic system has failed, when the user press the brake it can trigger a failsafe mechanism that really "reverse the polarity" in some degree (it is an AC motor so is more probably about controlling the phases and voltage). Even if you put the motor in shortcut (equivalent to MAX REGEN possible but not real) I don't think will be enough resistance to produce a decent quick stop, "reversing the polarity" doesn't mean the motor will spin backward but the two forces will counter each other and will decrease the speed in a controlled manner. Similar principle than ride a steep downhill on a low gear on a ICE engine.
Interesting problem !
Stopping a car travelling at, say, 70mph, when the brakes have failed, involves dealing with a lot more energy than the regen system can handle.
We know that the Model S with one driver on board weights approx 2,200 kg.
The kinetic energy at 70mph is KE=0.5 x Mass x Velocity ^2.
This works out at 1,077,659 Joules. That energy must be dissipated or stored somehow in order to bring the car to a halt.
If the process takes approx 4 seconds to grind to a halt, the rate of energy dissipation is (loosely speaking ) 270,000 joules per second, or 270kW.
But, in reality this is not a linear function, more exponential, and usually its dissipated as heat into the brake discs with a peak rate more like 400kW.
The maximum Regen of the Model S is shown as 60kw, not including various losses along the way. So, assuming the car was doing 70mph and you went into max regen braking slowly to a halt, it would take the Model S regen system (loosely speaking ..) about 15+ seconds to come to a halt (includes some losses to rolling resistance, aero losses etc). Next time you're driving try it out !
However, electrically storing or dissipating 400 kW momentarily is altogether a different problem to solve with some fairly beefy electrical engineering required.
This is one area where an ultra capacitor assembly could be used to harvest the energy. Based on my direct experience of using large ultracaps, I would guesstimate the weight penalty for the car to have a ucap regen system would be around 40-50kg, and probably add $7,500 to the cost of the car! But the major plus point of doing it this way would be that 95% of the harvested braking energy could be re-used the next time the car accelerates, or it could possibly be augmented into the energy of the battery pack. But if Tesla were to include Ucaps with the battery system, it would require some significant changes to the way the energy system works, and the load levelling effect would de-stress the battery pack and lengthen its usable life.
Whilst not simple in practice, it can be done. China now has thousands of ultra capacitor buses that spend all day accelerating / braking between bus stops a mile or two apart and they are amazingly energy efficient over the course of the day.