Found it!
I skimmed the source code included in the downloadable zip until I saw mention of something I hadn’t encountered in the game yet. This assured me that I just had to delve a little deeper and re-explore.
I had missed a box under the benches in the Guardian Eye room. In it I found a bottle of witches’ perfume. In one of those twisted associative leaps that go lightning-crackle in your brain, I remembered that there was another living being in the dungeon that I had encountered in a previous session. Under the bed in the Witch/Dungeonkeeper’s room is a black cat that only trusts you when you smell witchy. Once I had befriended the cat, it helped me as a counterweight on the second panel needed to lower the boat.
And now I’m free and weirdly hungry…
Great game!