> Arch actually has a guide to do it from within the other Linux, which seems crazy and fun.

It's not as crazy as it sounds, after all installing a Linux system is a matter of populating the rootfs, and passing kernel+initrd images to the bootloader. Just regular file system operations, apart from actually installing the bootloader to the MBR.

You can install Debian-based distros the same way using the "debootstrap" script.