Well the spacing on my forks is about 80mm, my Vee mounts on the back are more like 90 something, so it would never really fit. I think thats because its just a mountain bike one, which is why it fits fine on the front. Rather than a trials specific one built for wider rims and stuff. So if the spacing of your rear mounts is more than 80mm, it wouldnt fit, you could file it out to 84 max id say, but otherwise it shouldnt fit. The only one i can see fitting the rear is the Echo one on tarty, its a bit more expensive, but im sure its worth it, just check your spacing before you buy one. Plus it looks pimp as.
http://www.tartybikes.co.uk/vbrake_boosters/echo_vbrake/c106p13.html