Well, unlike the player image, there's no copyrighted data that would need to be included in a builder image, so this should be doable. Let's see. You'd need a kernel that had its root image on a ramdisk, the ramdisk itself containing a simple binary to do the following: create the partitions, turn on the swap partition, create the music filesystem, and burn in the filesystem, if desired. (I don't think we have to create the root filesystem, as the upgrade does that for us.) So the total things you'd need are the kernel, that init binary (or sh and a script, possibly), fdisk, swapon, mkfs, and dd and rm (or something to burn in with). Not a piece of even remotely copyrighted software there.