I would have to guess that ABS DOES work in reverse although I have never read anything one way or the other (actually I think I would have read it if it didn't). My guess is based upon the many false ABS activations I have felt in reverse. I can do it almost at will backing off my alignment ramp.
I would say that the threshold for activation is a mapped function. This means that there is a formula for when, based upon the circumstances in existance. Different thresholds are evident while cornering. Its possible the criteria would be different in reverse.
Bosch Master, ASE Master, L1
33 years MB technician