Does ABS work when sliding backwards?

I don't doubt that modern ABS and traction control systems take into account the many things you are referring to, but 1) I do not think early ABS systems were that sophisticated, 2) from a theoretical standpoint nothing more than a single wheel speed sensor is a requirement (at least in my current thinking derived from this thread), and 3) I observed multiple rigs slide backwards 20-50' with no evidence of ABS activity.

I think it would be hard to perform a similar experiment in forward. Gun it up an icy hill in reverse and the slide down forwards?

 

First and foremost ABS is not designed to prevent a or any wheel from locking up. The only way to make that happen is to not apply the brakes. What ABS does regardless of its name is: it measures wheel speed, compares each wheel to the others and if a wheel is rotating at a rate slower compared to the others than parameters built into the system want to see it will stop brake fluid pressure increasing at that wheel and if that is does not correct the situation it will relieve some pressure and repeat if necessary. If the friction between tire and surface is so low all four wheels could stop rotating and the ABS would just see that as the vehicle is stopped. Best thing to do is release the brakes and let the wheels roll and do your best to steer out of the situation. Any type of traction control is basically the opposite. If a wheel is rotating to fast the system may apply the brakes to that wheel.