The original Mechanismo plot is, admittedly, pretty thin. Some nice ideas that could be spun off into something more interesting, sure; or you could just jump right in with Wilderlands, which followed on from the earlier tales of robo-Judges. In this mid-90s arc that appeared in both 2000 AD and the Megazine, an increasingly erratic Chief Judge brings in the robots in again, much to the consternation of her peers - Dredd included - who reckon she's losing it. So, naturally, she gets all the people who wanted to take her down shipped off to a prison planet. Except that doesn't work out either, since said ship gets shot down on an unmapped part of said ship and said peers have to make their way through harsh alien terrain. Naturally Dredd takes charge, albeit without his badge. Wilderlands is basically one part Dredd to two parts Pitch Black, which isn't anywhere near a bad thing. You could even mash things up and make the desert the Cursed Earth to keep things earthbound, whilst still exploring how messed up the justice system is, how badass Dredd is and also robots.
Tom Baker is the Comics Editor at WhatCulture! He's heard all the Doctor Who jokes, but not many about Randall and Hopkirk. He also blogs at http://communibearsilostate.wordpress.com/