i seem to remember that Daniel Gawthrop posted Advice to
Choral Composers and Advice to Instrumental Composers some time
ago. Not sure if this is archived, but I'd like to be able to
find these resources.
I can't find anything like that in ChoralNet's archives. We do have some archived participation by Mr. Gawthrop (notably this discussion) but it's a bit too specialized to be called advice. I did find a request you made in 2004 for this same thing, but it seems you didn't get an answer then either. Maybe you should write to him and ask.
There is a resource on the site with advice for composers who wish to get published, but it is by Fred Himebaugh.