As you describe it, I would say it is not as conceivable as Mr. Mendes has presented it to be.
The first point is that many experts only consider subsection 41(a) of the Constitution Act, 1982, which deals with the office of the Queen. Everyone agrees that the office of the Queen cannot be changed without a formal constitutional amendment.
I go a little further than that, as I believe the prorogation power itself enjoys constitutional protection as a component of the separation of powers. Without even talking about the office of the Queen, I believe the power to prorogue Parliament, as a discretionary power and prerogative, enjoys constitutional protection.
If that is the case, it means that no significant limits can be placed on the Crown's discretion.
The restrictions you referred to earlier, such as preventing prorogation from taking place--