The cockpit isn't airtight - it would be at the same pressure as the rest of the cabin.

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.