Ville,
You bring up a good point on delay time. The best solution always depends on requirements. With some effort Tasker can do everything listed in the original post with several caveats, one of which as you point out is reaction time. Tasker would be great in an automation scenario where you could not only beep, but actually speak instructions to the operator based on different voltages, currents, resistances, etc. If the requirement is lowest delay continuity beep, a piezo buzzer as you describe would be simpler/cheaper/easier. —- Duane