[trebbble]

Boot from external media (USB disk, cd or dvd if you're kicking it old school, maybe even network) with no persistent storage (ramdisk or whatever only). Mount the hard drive. Format. Rsync the files over.

That's pretty similar to what installation media does anyway, really.

[EDIT] Oh you might need to install the bootloader too. But you can sort-of (your running kernel will still be the installation media's) do that from the rsync'd system once it's copied over, with some creative chrooting and mounting. Which, again, is something that Linux installers kinda do anyway, in ordinary operation, but you'd likely want to do it manually in this case. It's exactly what you used to do (probably still do?) installing Gentoo, even for a "stage 3" (least-painful) installation.