what happens if you lose your barcode?

Guess you better have terminal skills

It could be called:
a) iBalls
b) Ball-Sight
c) iSack

Hence my suggestion of a tattoo.

If you examine your startup script(s) (I don't mean scripts you install to run at start, I mean the one which governs the boot process on any Mac: /etc/rc), you will find a few references to password files. I didn't bother to work it all out, but I think it copies a password file from somewhere which I guess it then uses to compare to your input in the login window. Someone with better scripting skills and a longer attention span than me could probably work something out that way, but here is an easier one:

If you can get the barcode scanner idea to work during the login window, then simply use the barcode scanner to write half of your password into the password box, and type the rest yourself. Easy.

Thing is all it takes is say you got a nasty burn or a scratch on your tat then you're screwed.
That;s why they use finger prints. Some systems have a backup finger print from a different finger on the other hand so if you lost the finger or hand you have a backup.

I think the best idea is a simcard like setup or the good old fashion passwords.