This might come a bit too late to help, but that pit is closed (temporarily) by hitting the button in the opposite corner of the room as you go past during the closing pit sequence. It will close that corner pit just long enough for you to step onto it as the sequence snakes past; it will remain closed as long as you stand on it, and there's another button next to the alcove to launch the sequence again so that you can get off without having to jump down.