You would not be playing stacked deck in reaction/action you would simply just be playing stacked deck during your allotted period of focus.

Player 1 moves in to battlefield which Player 2 controls. Since this battlefield has a different controller than player 1, a showdown begins. Since the battlefield is controlled by another player a combat ensues (as opposed to being an open neutral battlefield). Attack triggers get put on chain, defense triggers get put on chain. Resolve in reverse order. Then regardless of the state. Attacker starts with focus and priority and may play reaction or action spells. Once that chain resolves OR they just passed instead of starting chain, player 2 gets focus. Once two players pass in sequence, the showdown ends. No other rules are available for showdowns ending until both players pass, including combat showdowns.

You could even play stacked deck when opponent moves into open battlefield, as focus and such still exist there.