After reading Rob's 3 month update, I went researching into sulfur consumption as well (I don't want to be deficient, but I also don't want to add something that isn't needed). The best info I've been able to find is here:
This paper suggests that a combined total of 3-4g per day of cysteine and methionine should be sufficient. The whey powder I'm using lists those at about 500mg each per 25g of protein, so at 100g of protein, I should be getting 4g/day, which should be enough.
The conundrum is Rob's reported deficiency. At 102g a day of whey protein, I would expect that he is getting enough as well. From what I've been able to find, oat protein should be about as good of a source of cysteine and methionine as whey, so even if Rob is reducing the whey protein to balance out the extra oat protein, it shouldn't be the cause of the deficiency.
One factor that I've been able to think of that might explain it is that the liver uses sulfates or cysteine to bind some pharmacological agents (the paper mentions acetaminophen specifically). Perhaps some of the nootropics require sulfur for the body to excrete them?
The other factor that might be having an effect is insulin excretion. Cysteine is a component of insulin, and so I wonder if large amounts of dietary carbohydrates might increase the loss of sulfur from insulin via the kidneys. Unfortunately, I have been unable to find anything to substantiate or refute this theory (i.e. there's a good chance it is complete nonsense), so please view it with skepticism.