The best fire suppression system is just plane water.  Before I retired we replaced all of our CO2 fire suppression systems with high press. water mist.  30 gal of water replaced 6 150# CO2 bottles.  The idea is the water will spray out in a fine mist and instantly turn to steam, this removes the heat from the combustion and also displaces the air.  I know that displacing the air would be hard to do in a moving aircraft but removing the heat would definitely put out the fire if the mist were spraying on and in the area of the exhaust this may prevent what ever fluid is leaking from re-combustion.  A high press. water system big enough to be able to do this would be weighty and take up a considerable amount of room (Water container + high press. gas cylinder plus high press piping and spray nozzles.)  I am not sure how to figure the  btu's stored in the exhaust system, but fairly sure that it would be a function of the mass of the eshaust system and the temp. of the metal, this would dictate how much water would be neede to remove this heat.  Water would have to be kept above freezing, this throws another kink in the logestics of this type of system.
Somone out there may have an idea on this and how to make this a practical system.

Bob P.