The million degree temperatures involved also might tend to be problematic (if the mass striking the plate is from close proximity to the detonation....). How would you armor the feed tube which ejects the bombs directly behind (and centered on) the pusher plate???
As I recall the bombs were to be thrown in from the outside edge of the pusher plate.
As I recall various materials were placed at different distances from the Bikini Atoll blast. A graphite sample deep in the fireball itself only lost something like an inch of it's thickness.