OK, here's a suggestion for sensing the bell (which, I recognise, isn't what you asked for, but might be a good alternative).
Sense the movement of the clapper by optical reflection.
Either paint a white spot on it, or attach reflective tape, somewhere that a reflective sensor can see. You may find that you need two sensors so as to avoid the problem where the clapper comes to rest just on the threshold of one sensor, and, despite hysteresis, you might find it's changing state frequently and causing false triggers; if so, position a second sensor such that it can't possibly be on the threshold at the same time, and look for changes (in either direction) from both sensors.
I'm still looking forward to hearing what you propose as the method to alert you, whatever the detector is.
David