I'm sure Urban is expensive. If they follow other judges and only use Dredd sparingly, I could see it working. If the show gets popular enough that they can afford Urban he could become a regular in following seasons.
I feel like if Karl Urban was in this, they would have announced it immediately in the press release to help promote the series as its main detail of interest. Seeing as how that's not the case, I'm struggling to see Urban actually being involved in this project.
Hope I'm wrong, though.