Several unrelated gnarly bugs later I finally got around to finish the patch to add a mount beneath the top mount. Sent it last week. IOW, allowing to atomically update mounts.
https://lore.kernel.org/all/20230202-fs-move-mount-replace-v1-0-9b73026d5f10@kernel.org
As usual all the fun was in mount propagation. It's currently called "MOVE_MOUNT_TUCK" but I'll rename to "MOVE_MOUNT_BENEATH".