After 47 hours of incubation the samples were all actively fermenting and beginning to show signs of exhaustion of starch supply. All of them had separated into a mat of gaseous foamy starch on top of a layer of relatively clear water and a layer of stringy, insoluble gluten on the bottom.
None of the samples containing rinsed and not-rinsed red cabbage appeared to be bubbling (evidence of the growth and respiration of yeast and bacteria) any more vigorously than any other. And none of the samples with cabbage were bubbling more vigorously than the control made with only flour and water.
All of the samples with cabbage had an "off" odor suggesting the presence of either bacteria or yeast that is either not desirable in bread starter or a desirable form of yeast that had begun to produce a noxious aroma (e.g. butyric acid). By comparison the control smelled like a typical batch of fermenting (proofing) bread dough.
All, except one of the samples with cabbage were slightly more acidic than the control. Cabbage starter samples ranged in pH from 4.5-4.75 with one sample testing at pH 4.90. By contrast the control tested at pH 4.8.
It is still too early in my investigation to draw any conclusions about the efficacy of making sourdough bread starter with cabbage. Nothing that I have seen so far suggests that the method does or does not work. However, the preliminary results of my little test suggest that adding cabbage to the starter may be introducing a microbe that can produce an "off aroma."
Since so many people have reported that starter made with a cabbage leaf produces great bread, I suspect that at some point during the build, the colony of microbes that is responsible for the "off" smell dies off.
As I wrote above, I'm a long way away from drawing any conclusions. I still don't know why or how this method works or, for that matter, if it works any better than more traditional ways of building bread starter.