I thought of using fill-flash -- unfortunately, at the time I thought of it, I was directly behind the fall, and my flash unit was a couple hundred miles away at home.

The only flash I had was the on-camera variety, and that, obviously, couldn't be tilted up; as a matter of fact, its angle of view would have provided illumination to the air a few feet of me dead center, while precisely failing to light the inside of the cavern.
There actually is a little bit of detail in the ceiling, which might be better seen in a larger image. I did try double-processing the file and manually blending-in the "ceiling," but, try as I may, the best result I got made it look like really bad HDR. Personally, I think the near-black looks better!