NOT a simple idea.
Even in the event of an inflight breakup it would narrow down the search area quite considerably.
Fuel discharged at altitude scatters over a large area and largely evaporates.
Just make the fuel traceable. Wouldn't be too hard/expensive to do it.
In the case of AF447 there was little trace of fuel. How would a marker within it have helped?
What sort of fuel additive could be produced in sufficient quantities to trace all fuel used in all trans-ocean flights, in sufficient concentration to show up immediately in a search, but without acting as a contaminant at any combination of environmental extremes?
No. A marker needs to be non-consumable, and close to the structure that is to be located.