well, I didn't have peroxide, but I did have physan, so I cut the leaf as far down as I could with some small scissors (sterilized of course) and then hit it with the physan. I let it sit a few minutes, rinsed it off, and then dried it and applied a good dusting of cinnamon. This morning (10 hours postop) it looks like the picture.
Also note that I discovered a new leaf inside the dying one...hope it can develop!
the affected leaf ended up dying completely and falling off, despite my aggressive measures. the rest of the plant just kept kicking, and seems to have been unaffected. weird.