If you examine the cat. you'll see a temperature sensor fitted to it,often quite close to the after cat lambda sensor(if fitted).The overheat warning light is triggered by this sensor.I recently renewed one on an HRV-only £190 from Honda! I suppose you could fit a dummy resistor to the sensor plug,say 1kohm to stop the light showing on cluster.This car may have old style diag. plug above n/s kick panel i.e. connect the terminals of blue plug to get flash codes,and a three pin diag. plug for Honda tester.