As far as I know, no aeroplane fire detection system allows any discretion not to make an emergency landing. I think few people would be happy continuing with a warning occuring. BA had a 747 fly for an hour with a cargo fire warning to get to Santa Maria in the Azores for a landing in a gale and a full emergency evacuation was carried out without delay. Should a fire have burst through the cabin floor, it would have been too late to then order an evacuation without fatalities, so it was an absolutely spot on decision. That made fascinating watching of the digital film from someone's movie camera during the incident, and the wind was very obvious.
Cargo holds on larger aeroplanes contain large metal baggage containers almost completely filling the volume. I don't see how a camera could film the range needed to adequately ensure all areas are clear when the fire may be located behind baggage or containers and the smoke blown towards the exhaust without being observable. I'm not sure if there are types of fire that produce fewer particles or smoke that may be undetected visually or by the smoke detectors. Ignore at your peril (and 400 other peoples' peril!)