I have a stock (mid-temperature) thermostat. What would be the temperature of the B230F engine block? Would I be safe considering anything above 130C to be overheating?

I'm thinking of getting a (cheap -- $0.45 at Digi-Key) thermal cutout switch (normally closed), siliconing it to the block, and using a spare relay I have to make it effectively a normally-open switch to drive a loud buzzer (an that unused indicator lamp).

I feel it's a much easier solution than replacing the expansion tank with one that has float in it, plus buying the level sensor. Although the warning may be a tad later in case of a slow leak, I guess that it's better as it will warn no matter what the reason for overheating is (it senses overheating rather than coolant loss).

Putting two cutouts in series makes you sleep even better, just in case one of the cutouts decided not to open.