I am sure you are aware that the MR2 is fitted with fire extinguishant when flying with long range tanks in the bomb bay. The precedent is there. BAe recommended it in 2004, how much time do you want?
I understand the arguments about increased volume and leakage rates when the tanks aren't there, but it can't be beyond the wit of an RAF engineer to come up with a workable design to see the aircraft out.
The time to design it for MRA4 is now. But it has not been requested by the customer.
Of course, safety is of personnel is of paramount importance.