you could easily have the joker still being involved without ever having to actually see him...in "the dark knight," he seemed to have his hand in all aspects of gotham's infastructure without actually seeing how how or where he set his plans in motion...plus, there are plenty of inmates in real life who continue to head gangs on the outside while remaining behind bars, so why wouldn't the joker be able to do the same thing while in arkham asylum?