Even if you start the stall demonstration at Max Cruise Alt (not service ceiling), the 'one knot per second' decel will mean the one g 'break' will occur at a somewhat lower altitude.
Actually, a deceleration of 1 kt/s IAS is 2 kt/s TAS (or more) at high altitude. Kinetic energy is a function of TAS^2. Getting rid of more than 2 kt/s of TAS at high speed often requires a climb for the first part of the stall test, even at idle. I've flown high altitude stalls where the stall occurred at well above the point where the deceleration was started (e.g. Global Express).