Dave:
Where did you get the information that there have been zero fatalities on Toyota trucks with SAS? While your comment does not explicitly say it, by the context of what you have written I am assuming you mean no fatalities due to accidents that are directly affected by the SAS, not fatalities in general on these trucks. The lateral part of the system merely locks up the rear axle to keep the truck from toggling, thus somewhat increasing the lateral g's necessary to tip the truck by changing from a stability triangle to a stability rectange/trapezoid. However, the truck can still be driven fast enough while turning to tip it over. The way that Toyota has advertised it in the past would suggest to some that the SAS will in fact prevent lateral tipovers with the lift truck being driven empty on a flat surface. There is some concern that lift truck drivers may actually depend on the extra stability and push the envelope, but I don't know that to be happening in the field. Of course, all of the other ways to tip over a truck laterally still exist (drop-offs, docks, unstable or severely offset loads, overloading, etc.).
Your comment about the maintenance of the system is very important, because IF there is a failure mode of the system where the truck converts back to the stability triangle ( I do not know that to be true one way or another), and it is undetected, a driver who is used to driving a certain speed in turns may in fact tip over the truck because he/she is relying on the system. Therefore, I would assume Toyota took this into account when designing the system, and hopefully made the reliability of the system so high, and the detectability of its failure so high, that this scenario is virtually eliminated.