Just some thoughts on sealing a bomb bay.
If it was not planned to seal the bomb bay then it would be reasonable to make them ventable.
The bomb bay is outside the pressure hull. The aircraft was designed to begin a patrol at 20000 feet and descend rapidly to low level to prosecute a contact. Once low it might open its bomb doors or not and then return to a higher altitude. Bomb bay pressurisation or even a sealed bomb bay would have been an unnecessary techical addition.
If, OTOH, it was desired to create an enclosed space with minimal venting and the ability to contain an extinguishant gas then I am sure doors without vents and with better seals could be fitted or retro fitted. They could be designed in such a way not to be pressurised but to maintain atmospheric pressure. As they have the aforementioned hot air bleeds they would obviously need vents. It follows that it would then need vent closures and well as bleed closures.
Extra additional weight and complication but it does not seem an impossible task. It would of course also need inlet vents to prevent underpressurisation in descent and exhaust vents to prevent overpressure in the climb.