I have no idea if the cockpit is pressurized separately from the rest of the plane, or if there's any way to tamper with the deployment of the plane's oxygen masks. It's all speculation on my part.
The cabin generators can be disabled by tripping a circuit breaker (and they only last about 12 minutes anyway).
The recommended limit around 43,100 ft. 45,000 ft isn't that far above it so it's possible that there may be no impact on passengers at that level (if you're saying 43,100ft is safe, you're going to have a margin for error, it's not going to depressurize at 43,101 ft).
Certainly if it was part of your plan to incapacitate the passengers it seems overly complex, risky and uncertain.
That said, given what happened on Flight 77, if you were hijacking a plane, a plan to deal with the passengers would make a lot of sense. I'm just not sure this plan to do so does.
Ascending to 45,000 ft or above is an aerodynamics issue, not a pressurization issue. Pressurization ("cabin altitude") will adjust automatically.
But that doesn't change the fact that flying at 45,000ft probably wouldn't do it and would be a very risky way of not doing it.
De pressurizing the plane would be extremely dangerous anyway, even with your oxygen mask on. Among other things the cabin temperature would drop to -50°C or so, the low pressure could induce fatal embolism, loss of consciousness, etc.