Thanks for the reply.
The camera adjusters seem to actually work quite well. I've used large washers so that when tightened, they force the piece of wood to correct position. They're mounted on the cradle through the large pieces of wood, so the hole is long enough to ensure that the threaded rod is at 90 degrees angle. The idea came to me while wandering around in the local hardware store and seeing the rods. That's one of the better parts of the build. I think I'm not going to move the cameras to the platen after all.
The base of this prototype unit is not like a keyboard stand, but it's definitely a viable option. I'm planning to make a small series of these to provide to other people of our hobby project, and those might very well be foldable to make them easier to transport and store.
Looks like I'll be getting non-reflecting glass (used in framing paintings) for next version of the platen. I've been pondering about the platen lift mechanism, and the current idea is something like a gallows with pulleys and wire to lower and raise it. I'll try to draw a picture of what I'm thinking, and how it would work. Optimally it would be controlled by pedal, so that pressing the pedal will raise the platen. Also, the gallows frame would serve as mounting point for lights. The supports for the gallows would only be on the back side of the scanner, so that the front is completely open to make page-turning and platen positioning easy.
edit: ...and yes, this one is rather solid. When doing woodwork, I seem to always do the same "mistake" of oversizing all load-bearing parts. I could've used way less wood and the end result would have weighed half of what it does now, probably. Well, at least it feels very solid and nice.