I'm with you, Medsen - too much guessing, and not enough data. To further muddy the waters, remember that although active dry yeast may be 52% protein by weight, that doesn't necessarily correlate to half of the nitrogen in yeast being trapped in unassimilable proteins. Additionally, part of autolysis is proteolysis, where some of the proteins in the cells slowly break down into amino constituents. SO... what you get from "brewer's yeast" that has been partially autolyzed in processing, may be significantly different from the YAN obtained by boiling "fresh" active dry yeast.
We clearly need to make up several samples of alternative "natural" nutrients, and have them properly assayed, before we can come to any definitive conclusions.
At least from what people have thus far uncovered here, my assumption of 12.5 ppm per gram per gallon (which was "gut feel" arrived at from an extrapolation of what is currently available from a number of commercial yeast nutrient products), isn't too far off the mark. ;D