From my understanding, app-alternatives/cpio is only used with an initramfs. For use cases without an initramfs, app-alternatives/cpio is not needed.
It is not practical to do this since cpio is on the upstream list of required software for building a kernel -- and for good reason. Depending on your .config, correctly building one of the kernel modules requires cpio -- that kernel module is enabled by default. It's not possible as far as I know to have BDEPEND be different depending on which specific modules you have enabled in your custom savedconfig or /etc/kernel/config.d fragments :)