Actually, I had thought of this as an experiment some time ago as a way of replacing the springs as a point of wear/failure on buckling spring boards - interesting to see others with the same idea, and the commercial board.
Might be fun to try to make it tactile as well as combining it with something more durable than membranes, like capacitive, hall or reed switches.
Along tactile lines, a cross-wise magnet in the zone of travel would create a tactile disturbance in the field, but it wouldn't be very sharp and you'd still have to figure a way to make it coincide consistently with the switch activation.
A coil that fires a magnetic pulse momentarily strengthening the field of the bottom magnet (or disturbing the field of the top, key, magnet) when "contact" is detected might be ideal, I think. Result would effectively be instant feedback of electrical "contact" and ONLY when a keystroke is actually registered by the logic circuits. Unfortunately, the power requirements would likely be painful.
Edit: See below...