Am I missing something? Why is it that you want to do this? April is quite a while away....
Set up a relay to drop out (open) from a relay - put it far away enough that you wont hear it. And control that relay via a magnetic reed switch like those used for door contacts in alarm systems, and attach it to the bottom outside of the tank of the toilet. Then get a fairly strong magnet on a string hanging from the spindle between the lever and arm inside the toilet close to where the arm and lever come together. Set it up so the magnet has just barely enough strength to keep the switch closed, but will open on the slightest jarring of the string. As an electrician I would say to keep the control voltage as low as possible. When the toilet is flushed the magnet wil move and swing around in the tank just enough to shutter the relay and continue to do that until the currents of water stop after the tank fills. And if you are electronicaly inclined do something simular with an SCR control circuit instead with an even lower control voltage. Or use the red and blue lv conductors of a relay like this:
http://www.wattstopper.com/getdoc/1224.pdf
