Adding dye bags, certification would only apply to current fleets then it would come fitted on new supplied frames.
Adding dye to fuel would need tests if not certification to every engine type, fuel tank & internal wiring, pipework and joint seals etc and as already pointed out quantity will diminish during flight, so how effective would be towards the end of a flight.
You are constantly using and renewing it, so cost are increased by mileage, may even be environmental issues.
KISS, passive systems less agro
"Fluorescein" would appear to be the dye of choice: a powder, widely available, medically safe and already used as a sea stain. Bags of that could be thrown into cargo holds (or elsewhere) tomorrow. The sort of crashes we've been discussing have disintegrative impacts, so dispersal from inside the airframe shouldn't be a problem.
The extra weight and bulk of the dye pack shouldn't matter since most airline flights are at less than MGTOW, and bags stow anywhere.