Exceeding Mmo?
Don't!
Or to put it in more detail, at MMo you will probably already have shock waves forming on the upper surfaces of the wings and tail. You might even have shock waves forming on the undersides as well. If you exceed Mmo they will start to move back towards the trailing edge and grow larger. As this happens the aircraft will experience a nose-down pitching moment that could eventually become too large to counteract; this phenomenon is know as Mach Tuck and can be lethal if an aircraft is not designed to deal with it, because the nose down attitude leads to a speed increase, leading to a greater moment, leading to greater speed and so on. It can become "unrecoverable".
You can also experience effects such as reduced effectiveness of flight controls, or even reversal of their effects, as the shock waves approach the ailerons and elevators.
So, if the choice is some or all of the above vs a loss of altitude to achieve engine relight IAS, I know which I would choose! And pressurisation loss should not be a factor; there are one way valves (NRVs) in the ECS to stop air going the wrong way and out through the engines, and on a modern aircraft it should not leak out anywhere else at a rate that would be cause for concern. Having said that, I was once on a VC10 K3 that was completely maxing out both cabin compressors just to maintain pressurisation at FL380. If we had lost one we would have been descending sharpish!