I wouldn't mind the piston thing, actually. Just find a way for it to work, and have the wall continue to be organized. Also, instead of using clocks, it could just be Staff Triggered. Someone complains about it, and a staff members pushes a button, triggering all the pistons. Problem is, as laithes said, these will cause some lag. Not too much if there is no pulsar/clock, but still lag. :/
Well just about anything in SMP can be a lag issue if done wrong, and triggering all the pistons at once would cause more stress than a slow cycle. but this is just a silly idea anyways. Another idea is a slow clock around the wall with netherrack on the edge, so a cycle of fire goes around on the top of the wall, catching people who loiter too long on fire.