It'd be interesting to make a full portable OS setup. This could support booting a disk or CD image through qemu or syslinux. For a typical USB stick or hard disk, this can be done via syslinux on the MBR or such; for an iPod, you simply have to remember to hit the second partition with your bootloader.
Consider first that the iPod has two partitions, both flagged as bootable. The first partition will be ignored because it's not bootable on PC; the second, however, will be booted. For this, we can syslinux the second partition rather than the MBR on the iPod.
Device Boot Start End Blocks Id System
/dev/sdb1 * 1 14 112423+ 0 Empty
/dev/sdb2 * 15 7296 58492665 b W95 FAT32
iPod 60GB 5th generation shown
All that's required is a utility to run SysLinux for Windows/Linux against the iPod's fat partition or the external hard disk's MBR; and to build the SysLinux and qemu settings.
For an ad-hoc setup, qemu could be configured to load live CDs and disk images from nearby directories. The more direct setup would allow the user to also boot off the USB device and load the full OS or disk image; but this would require boot-time awareness.
The most particular consideration of portable Linux operating systems would be that the OS must at boot time be aware of the potential to have to find itself as a file on other media. It would be possible to pass this information to the host OS if it could be passed to the boot loader on chainload. Perhaps this should be worked on with some Linux OS developers, maybe from Ubuntu or Gentoo.