I've recently had reason to research some of the law and conventions around Cabinet appointments. There's been plenty of innovation since we adopted MMP. 

We have Ministers who are not part of the Government, Ministers not bound by collective responsibility but no MInisters outside Parliament.

The last is a reflection of what we think as a core element of the Westminster system – that the Executive draws its authority from being the faction in the legislature that has the numbers to call the lawmaking shots. Being able to appoint a Minister who is not an MP is thought of as a characteristic of US style administrations, where the separation of powers divides the executive from the legislature.

The UK PM has just announced the appointment as a MInister of the current Chairman of HSBC. He'll get a seat in the Lords, so strictly he will be a Parliamentarian. Peter Mandelson was the last Minister similarly parachuted into both the Lords and his Ministerial warrant.

But it underscores a flexibility that we do not have in our Constitution.

Section 6 of our Constitution Act 1986 requires that Ministers of the Crown be members of Parliament.