Make a lemming a rock climber and time it so that he falls directly onto the chain, without jumping.  He needs to be released such that he is flung all the way to the right side of the level and bounces off the right wall (keeping him from climbing that wall).  From there, jump over the gap and pray that he drops off the edge just as the chain is arriving; you can assign the jumper early and climb up a bit of the side of the gap to help make the timing more favorable, but it still takes some finagling with the chain speed.  Release the lemming to the left and have him laser blast to the far left edge of the crowd's holding area.  If luck holds, no right-facing lemmings will be directly over the laser when it breaks through, and everyone will walk left to the exit.