I don't know of a real
distribution for this. That's kind of overkill by definition. You don't need a package repository, you don't need much of anything at all, just a kernel and a bundle of files.
See
initramfs on how to make that bundle of files. You'd probably
use busybox in it -- it's a single swiss-army-knife executable which you can use like
busybox cp source destination if you want cp,
busybox cat filename if you want cat, etc, etc. Be sure to have a symlink /bin/sh pointing to /bin/busybox so scripts can run.
Just put whatever script
you want into
init instead of the stuff you'd use to chroot into a real system.
Going back to grub isn't trivial. You can't just "quit linux" and go back to grub. The
kexec option allows you to boot another kernel, which would allow you to boot grub raw again without rebooting, but would take some fiddling to manage. The quick and dirty way to manage it is, of course, to just reboot...