I've been struggling for a couple of weeks to decide how to end of this piece. I finally took a "just compose" course and started writing out notes. Soon I realized I was doing something like Shepard tones, so I thought, "Maybe I'll do this for awhile and then end it." Then I went through everything I had written up to this point and timed it. The passage shown below corresponds approximately to the jagged, angular section in the sopranos, beginning around 13:30. It ends about 15:30, but is just beginning to pick up some momentum at that point (haven't filled in everything yet in this picture). I began to realize that this could be the way I build to the end. It's a simple way to gradually layer repeated figures to thicken the texture. The timing is almost perfect. But the best thing about the Shepard tone* is its representation of circularity. Considering the selection of the text with its circular form, and that it will become intelligible as language during this passage, I think this is a bit of serendipity to have happened on the Shepherd tone.
*) Of course this isn't a real Shepard tone, but it is a stylized approximation similar to the one found in Ligeti's etude "The Devil's Staircase."