Farknell. The discussion was about simulated engine failure after take off in a light single. Typically the throttle is closed to idle at 500 feet and the glide commenced. At 200 ft a go-around is made.
The total time in the glide is around 30 seconds. Hardly time for the engine tempertaure to drop 10 degrees C - not exactly "shock cooling" stuff. In any case there have been many earlier Pprune discussions on the subject of "Is shock cooling a Myth".
You don't see much evidence of aircraft falling out of the sky when heavy rain cools the cylinders in a most effective fashion.
But let's not go into that until another day.
As one who spent time at Point Cook a long time ago, I can assure readers that the skies were full of Wirraways and Tiger Moths with countless practice engine failures after take off. We coped very well considering that the Tiger Moths did not have radios. All done by the simple expedient of gently closing the throttle, lowering the nose to best glide speed, selecting a likely landing area (ground or sea) and then a normal go-around at 200 ft without throttle bashing. I do not recall significant engineering problems when engines came up for inspection. And hopefully no one will come back at this post with the hoary old point of all that happened a long time ago and is not relevant to modern aero-engines. Airmanship principles do not change with the years.
Let's keep things in perspective. Aero-engines go through stringent certification testing before acceptance This includes a day running at full power, then lots of deliberate mis-handling. There is no need to have heart flutter over a closed throttle 30 second glide, believe me.
Sunfish. "Warming" the engine by opening up to 1500 rpm for 10 seconds will make no measurable difference to the engine temperature. The purpose of the exercise as you describe is to ensure that spark plug fouling is minimised in a prolonged glide over several thousand feet and in any case the power should be increased to full throttle to ensure this power is available when needed. Leave the carb heat on while so doing - this minimises carb ice build up.