This is not possible without modifying the kernel source code. And even then, it is only a theoretical possibility. This is because the root filesystem is pre-mounted as "root_device" by the kernel, prior to starting
init(8).
You can, however, make the root filesystem physically read-only. Many users have done this over the years. For read-only IDE/ATA or SCSI attached devices, only /etc/rc need be modified. For an optical root device, the kernel will need a custom configuration, too.