Given the amount of people that are consuming large amounts of this stuff and reporting their progress, I'd have expected any serious nutrient deficiencies to have been reported quite a while ago.

Absolutely not. You can be deficient in some mineral for your entire life, with the only thing to show for it a permanent light rash and an increased chance of colon cancer. E.g over 50% of the population is deficient in magnesium, even following conservative daily suggested intakes.

People report Soylent 'curing' rashes: that's exactly the opposite happening. They were deficient in e.g. zinc until now and finally got enough through Soylent. A good multivitamin would also have solved it.

Multivitamins are starting to get the stink-eye from medical science [1][2]

There's argument to be made for supplementing a known deficiency, but it seems as if you're better off eating it in food rather than a supplement.

[1]: http://annals.org/aim/article/1789253/enough-enough-stop-was...

[2]: http://www.theatlantic.com/health/archive/2013/07/the-vitami...

Is there a test to find out whether you're deficient in Zinc, Magnesium etc? Tailored vitamin/mineral supplements could be a pretty cool startup.

I'd suggest that you read research on the topic instead of having expectations. Some things only affect some people. Other things take a long time to develop.