Thanks to everyone here. I'm a bookbinder who does some conservation work, and I usually have to scrounge/build my own equipment so this board is a great discovery. There was a link here on the Book Arts mailing list today in response to a question about book scanners. I'm just getting started researching and reading to make my own scanner but I hereby venture an observation regarding the use of "Gorilla Glass" to hold book pages open: I'm afraid of it. I'm very afraid. This glass has been treated (similar to a fluoridating a tooth) so that the surface is stronger and less porous and less likely to break. But it's still not tempered, and if it breaks it will break into sharp shards. This is very bad for a paper book. The pages might be cut by the shards either on breaking, or in cleaning up the broken glass, and tiny pieces of glass shrapnel will remain in the gutters of the book or embedded in the page surface, and continue to do damage over time. My guess is that if a book is worth scanning, it's worth not sprinkling with broken glass. The chance of breaking the glass is small, but non-zero. Even tempered glass will break and create sharp edges and microscopic shards which can lurk around and get on other books. I hate broken glass.
I think of books as precious objects. Any book that's made it through hundreds of years of history (try that, 5 1/4" floppy disk!) deserves respect, even if this can border on obsession. Or paranoia. Or paranoid obsession. You get the idea.
I use some 3/8" thick plexiglass (Tap Plastics/US Plastics) as pressing boards for oversize books, and these are very strong, so I'm leaning toward using this. Plexi can break, but the pieces tend to be more rounded and not so fragmented as glass. It's cheap enough to replace if it gets scratched. Hooray, another pile of stuff that I didn't even know that I didn't know! Thank you, ignorance, for another chance to enjoy learning something new.