A command line linux would even fit on a bootable floppy disk (but my last 3 generations of computers did not even have one -- just that it COULD). Like "Tom's Root Boot" for example:
One problem I can see with a minimal distro is supporting all the different USB interfaces, and onboard USB hubs. Sometimes trying to initialize them can lock the computer if not done in the correct order...
Going with something a bit bigger (like Puppy Linux) may be more likely to work out of the box on more hardware.
EDIT: And then there are pesky problems with USB boot device resetting and losing the boot drive, or SATA CD/DVD support issues, etc. Oh, and max karma from me too.