So I finally tried it today. This is how it works: in a big pan of water about 1/2" deep, with a soft brush lay the ink on top of the water, and swirl it with the brush or with something else. Lay fabric on top for about 10 seconds, then lift the wet fabric off and lay it on newspaper to dry. That's all there it to it.
Yesterday I prepared some fabric with Bubble Jet Set, assuming that the sumi ink would be akin to dye-based inkjet ink. The first three prints shown here were done on the BJS treated fabric. This morning I reasoned that because the sumi ink is made from vegetable oil soot, it's probably more like a pigment ink, therefore I probably wouldn't need BJS. So I also printed some muslin without the BJS to try it out. Although the ink finished as permanent in either case, I like the prints better with BJS.
In the tutorial I mentioned, it says you can add color to the water plate with oil paint thinned with linseed oil. I didn't have any oil paint in the house and anyway, I was most intrigued with the black and white prints. But there's obviously a lot of potential for creating really unique patterned fabric with this method.