You might look through some of the past SANS and USENIX preceedings... I know I've been in a few discussions re:steganography and the like (more likely at USENIX) in the past few years at one of those -- you can probably get a few of the essays or conference notes on it... if you can't find anything, PM me or something and I'll see about going through some of my past notes for you.

...though, and you probably knew this... finding watermarks and stego's in pieces of work is generally, well... part of the premise is that a "good" piece of stega blends in so well with the original document that it's virtually undetectable unless you know exactly what you are looking for, etc...