Some progress: I've confirmed that monster hypercharging is a real glitch (and have documented it at
Wiki: GameResources/DOS/Nethack#MonsterHypercharging). The setup that I used for the tests was to be in the a corner of a room, with a hole next to me, and water on all the other adjacent squares and below me: a dragon is flying over the water, with my character in homunculus polyform (which is speed 12, able to fly, and bad defensively). Waiting the turn would cause the dragon to attack the character, rehumanising it from HP damage and causing it to climb out onto the square with the hole, immediately falling through to a lower level. The monsters on the level that I fell onto would gain a turn's worth of actions, and if I immediately level-teleported out and repeated the process, the monsters would gain another turn's worth of actions, and so on. (I also confirmed that, if I waited on the new level rather than level-teleporting out immediately, the monsters would immediately take all their stored actions.)