This is how I plan to solve this problem on the scanner I'm currently designing.
The cradle is made up of three pieces: the spine support and the two sides. The sides are attached to the spine support with a sort of hinge (long 3/16" hinge pin). The spine support can also rotate about a long hinge pin through its center (longitudinal axis) that is also attached to a tab off of what I refer to as the cradle base at each end.
At the top of each cradle side there are short link arms that attach to the side and to a pivot point on the cradle base. The width of the spine support matches that of the book you wish to scan. The length of the short link arms is the same distance as half the width of the spine support.
This configuration results in each of the cradle's sides as being the long arm of a parallelogram. As the spine support tilts, the cradle sides also move but the angle between the two sides will always remain the same which ensures that the platen will contact the pages properly throughout the scan process.
This does require that the width of the spine support member change depending on the thickness of the spine of the book being scanned. That can be addressed by having a number of holes in the spine support end piece and moving the hinge pin to the proper set of holes. You could also do it with a rod and setscrew setup or maybe a sort of turnbuckle. The length of the short arms would also need to change accordingly and can be addressed in the same manner.
One thing to note is that while the short link arms hang over into the area inside the cradle sides, the platen will never hit these because the thickness of the book extends off of the cradle side farther than the end of the link. When there are fewer pages on one side of the cradle, the cradle side becomes collinear with the short link arm on that side. Kind of cool how that works out. Of course you could also make the platen so that the glass would never extend beyond the cradle edge.
Enough talk. Here's some photos of a mock layout I made on some poster board with brads as hinges. The spine support and short link arms are red. The cradle sides are gray. The cradle base is outlined in black on the white poster board.
Pic 1: General layout (middle of scan position)
Pic 2: Scan start position
Pic 3: Scan end position
Pic 4: Scan start with book
Pic 5: Scan end with book