I ended up using Mylar type D because silicone release paper was not readily available and because mylar can withstand temperatures down to minus 100 degrees F without getting brittle, and will not begin to soften and melt until temperatures are in excess of 400 degrees F, it is highly durable, dimensionally stable, reusable, inert, smooth and clear. All these things seem to work in its favor, and I never experienced any ink transfer with it.
Regarding temperature, I found it depends on the age and history of the book. I realized significant ink transfer down to 125 degrees F on circa 1970 books with low cover gloss prior to my use of mylar. Conversely, I took a glossy silver age book up to 180 degrees F for a significant period of time with no ink transfer.