Page 11, in the 2nd box it states:
Jehovah's Witnesses and Health
"Jehovah's Witnesses, some of whom are physicians and nurses, are known worldwide for their rejection of transfusions involving whole blood or primary blood components. Does their united stand against this practice stem from man-made doctrine or a belief that a person's faith can heal medical ailments? That is far from the truth."
Many HLC brochures are not published by the Watchtower, which gives them a "way out" of having to take responsibility.
On this page at the bottom it states:
The medical section of jw.org is designed as an informational resource primarily for use by clinicians and other health-care professionals. It provides neither medical advice nor treatment recommendations and does not substitute for an appropriately qualified health-care provider. The clinical literature cited is not published by Jehovah’s Witnesses, but it outlines transfusion-alternative strategies that might be considered. It is the responsibility of each qualified health-care provider to maintain awareness of new information, discuss options for care, and assist patients in making choices in accord with their medical condition, wishes, values, and beliefs. Not all listed strategies are appropriate or acceptable to all patients.
This might be some good evidence, but I would still feel better if I could get my hands on a letter. Getting a copy of that letter is going to be very rare.