I have promoted AAA players to the ML for the playoffs and then demoted them when the ML team has been eliminated. Never got a demotion penalty.
I've come to believe that playoff games do not provide much of a performance boost. Especially when you factor in a player could get a major injury. For that reason, I would not test this on a top prospect. If you want to learn what happens, demote a few career MinL players this way and see what happens.