Can the sulphur content of biological material be reliably calculated from fat, fibre and protein contents (Weender Analysis)?
I would say theoretically yes, if you know lots of other information, but practically no, because of all the unknowns.

According to this paper cysteine content and therefore sulfur content of proteins vary depending on what kind of organism your organic material is coming from. Animals have more, bacteria tend to have less.

The sulfur content as a fraction of protein content in a given animal or plant is constant or nearly constant across the entire animal/plant. If a sample's protein content is 50:50 corn protein:sheep protein it's possible to estimate the sulfur content of the protein fraction from the sulfur contents of the sources of the protein. 

In addition the sulfur content can be affected by fermentation and sulfur/sulfate-reducing bacteria, many but not all of which are anaerobes. So figures for whatever the source of the feedstock is (as far as cysteine content) can be affected by whatever happens to it afterwards. Chicken waste with a high sulfur content allowed to ferment may give off fairly large amounts of hydrogen sulfide, lowing the sulfur content without necessarily affecting the bulk protein content. The reverse might also happen, where protein is digested and sulfur is released as sulfates but kept in the mixture. 

Even if you know for certain the sources of all of the protein(and that no further biological activity has changed them), the unknown amount of organic acids is going to throw off your sulfur estimates if there are sulfur-containing organic acids in the N-free fraction.