The water-push trap sounds great, and I think it could work real simply like this.
╔══╦════════╗
══╦══╣≈≈║≈≈≈≈≈≈≈≈║
++┼¢ò┼≈≈╬≈≈≈≈≈≈≈≈║
══╩══╣≈≈║≈≈≈≈≈≈≈≈║
╚══╩════════╝
╔,═,╗,╣,║,╚,╝ = wall sections
≈ = water
┼ = door
¢ = hatch cover
ò = lever
╬ = single-tile, unraised bridge of the raising type
The lever opens the hatch cover & the second door, it also raises the bridge. The open hatch cover both prevents the noble from leaving and provides access to the stairwell shaft below. The open door allows water to wash in and hopefully shove the noble down the hole. The raised bridge keeps the water tank from refilling from the larger water supply beyond
This water should not be pressurized, so that it slides sideways. That's the only way water will move other objects.
The whole thing will need to be reset, somehow. One option is another door off to one side of the lever, that is usually locked. The only time you unlock it is when another dwarf needs to go in and pull the lever to close the door and hatch cover. Then you wait for the water to run out, unlock the side door, and send a dwarf in to reset the lever.
Now, the second tank is a bit of a problem. It can't be pressurized because that will pressurize the fist tank during the 100 steps it takes the bridge to raise. Also, it would be best if both tanks had somewhat less than 7/7 water. For this reason, and to refill the tanks once the door is closed and the bridge is back down, I'd recommend that the second tank be kept partially filled by something like that one repeater that, as a side effect, can keep a body of water at 3/7 - 4/7 automatically.
Alternately, instead of that unraised bridge of the raisable type, you could keep the first water tank connected to a pressurized water source by a door. Then you have a dwarf pull the level to close that door before you send the noble to pull his own special lever. This depressurizes the first tank. When you send a dwarf in to reset the suicide lever, you can after ward send a dwarf to open the door that leads from the first tank to the pressurized water source, to refill the tank.
Damn, this seemed so simple before I started writing it down. Hopefully someone can come along and make this more elegant.
EDIT: on second thought, the unraised bridge of the raising type could indeed be all that separates the first tank from a pressurized water source. In the 100 steps before the bridge is raised, pressurized water will flood the tile on which the noble and lever sit and will not push the noble anywhere. However, once the bridge closes the pressure is cut off and the noble gets washed down the hole just like he or she would have before.