Update. Still treating with supplements. Every day I move some out of the hospital brooder because they have recovered. I am not seeing any new cases. 5 out of 20 have died and I expect a few more. My Light Brahmas are by far the breed most effected, followed by Rhode Island Reds and Barred Rocks. Although I had 100 Easter Eggers and 50 of each of the other breeds, Easter Eggers were not affected at all. Possibly because they are a hybrid?
The symptoms seem to indicate a botulism outbreak although I have not had anyone tested. Gail Damerow's The Chicken Health Handbook even describes this poisoning as "resembling Marek's".