Why not disconnect the plug and attach a new loop circuit to an axtra switch under the clutch ?
That way pressing down the clutch would make the circuit and "hay presto" the beeper would work !
That way you will be reminded only when you forget, and only when you go to drive off ! If you didn't put your belt on, you would be reminded at every gear change!
I think this would fulfill all of the required safety features and also stop the annoying Beep in a very simple fix.
