Erm, why would you want ABS in reverse, wether its in LFS or real life... Not like you race backwards... In real life maybe in snowy conditions it can be useful (even then lockups are easy to get) but in LFS where you're sopose to be racing or whatever I doupt you need ABS in reverse O_O.
Wait, after thinking about this, I just realised that it happened to me sometimes that I crash or something and while backing up and braking I lock up. But meh, not really THAT important.