Cotton is now being genetically modified into a major source of food for the future world population, according to this source. I'm not a fan of GM, but cotton seeds, cotton seed oil, and cotton seed flour is already being used in food. A cotton plant produces more (in weight) cotton seeds than cotton fiber. I've already seen cotton seeds (or was it it cotton seed oil?) mentioned on something I've bought (AFAIR it was some Indian curry paste) in the local grocery store.

Here's a quote from Cotton as food in second green revolution:

“It’s obviously the greatest source of fiber in the world,” Cotton Incorporated researcher Robert Nichols says in the video, “but there are products we can make for human consumption from cottonseed. We need to think more of cotton as a food plant.”

Keerti Rathore, plant biotechnologist, and his team at Texas A&M University, have developed a new variety of cotton that has no gossypol in the seed.

“Every year, the world produces about 44 million metric tons of cottonseed, which is equivalent to 10 million tons of protein,” he says. “People like the taste of cottonseed; many would much rather eat cottonseed than soybeans.”
Now, what about the B12 levels in cotton?

3. Ordinary commercial cotton contained 1500 μμ g. B12 per g.; cotton from cotton bolls, 700 μμ g. and absorbent or non-absorbent, 10 μμ g. or less
1500 picogram/g equals 0,15 mcg/100g. I haven't seen studies on B12 levels in cotton seeds, and don't plan to eat a lot of cotton derived anyting, but it's interesting with all this information popping up about small B12 amounts (and we need really small amounts of B12) in plants, water, soil etc.

The full abstract:

1. Vitamin B12 activity for a number of natural products was determined by the growth of Euglena gracilis var. bacillaris. 2. Variability of Euglena and the presence of inhibitory substances in extracts of natural materials influenced the results of assays. 3. Ordinary commercial cotton contained 1500 μμ g. B12 per g.; cotton from cotton bolls, 700 μμ g. and absorbent or non-absorbent, 10 μμ g. or less. Yellow bone marrow contained 0.002 μ g. per g.; shrimps per g. fresh weight, from 0.0055 to 0.009 μ g.; oysters from 0.2 to 0.460 μ g.; clams, from 0.14 to 0.18 μ g.; earthworms, from 0.08 to 0.180 μ g.; marine algae from less than 0.0001 to 0.1 μ g. per g. dry weight. Oyster juice contained per ml. 0.01 μ g. and clam juice 0.0014 to 0.010 μ g. On a dry weight basis earthworm casts contained about 2 per cent as much B12 as the earthworm. The B12 activity of an extract made from fresh clams was increased two or more times by heating with 0.1N HCl. 4. Blue-green algae grown under sterile conditions in a medium free of B12 contained up to 0.64 μ g. per g. dry weight. Blue-green algae have the ability to synthesize substances with vitamin B12 activity and are considered to be one of the primary sources of this vitamin for water animals.
Don't start to eat your T-shirts quite yet. AFAIK, cotton (at least the seeds) needs to go through a refinement process before they can be used for eating. Lots of food contains elements (eg. nutrients) that may be toxic in large amounts, but which are beneficial in small amounts, and I don't know how much one would have to eat of something derived from cotton seeds before the effect was negative.

More about the toxic effect of too high amounts of something that's good for you in small amounts here.

I haven't googled how much an average adult eats per day in grams, but if we loosely assume that an adult drinks and eats a total of 2500g per day, and we should consume 2.4 mcg B12/day, this would mean that we'd get enough B12 even of what we ate and drank only contained 0,001 mcg B12/g - or 0,1 mcg/100g. B12 in such small amounts per gram is also absorbed efficiently.

If Victor Herbert's statement about most people not needing more than 0,3 mcg per day (again, I assume this only would be valid in an unspoiled, 100% "natural" world, so forget the validity of this statement for people who live in this century!), less than a third of the above estimate would be enough. We're talking about less than a third of 1/1,000,000 gram here - an amount so small that it probably belongs in the 'hard to detect' category. Not only that, but several observations suggests that vegans actually need less B12 than meat eaters, and in his original writing, Herbert actually wrote that 'the minimum daily requirement (MDR) for vitamin B12 to sustain normality is probably in the range of of ~0.1 mcg', in other words, one could possibly get enough B12 from drinking water alone, without eating any B12 rich food, if living a few hundred years ago on a healthy, balanced, organic (there were no non-organic options back then) diet.