I chuckle at how one auction often inspires others:
http://cgi.ebay.com/Vintage-Macintosh-Plus-800K-Yellow-Disk-Drive-Protector-/140437262079
But I will give Dan credit though. He is better educated than most on EBAY, and if you examine his listing closely you can see he is very careful to only associated the yellow disk with 800k drives. I applaud that because there is no hard evidence to say that the yellow disks were used on the 400k drives that shipped with the 128k and original 512k machines.
Once again, my memory is not perfect, but I have no recollection whatsoever of such a disk insert, yet I recall other things about my first Mac very vividly. My father brought the Macintosh 128k home in 1984, new in the box. I was 13 at the time and was given the green light to open the box. While setting up the computer, I recall many things about how I examine the Macintosh computer closely, top, bottom, front, back, sides -- peering inside the floppy drive slot, in the slits at the bottom sides and even wondering what those slits were on top. I connected the mouse, keyboard and had lots of fun with the Guided Tour floppy.
My memory of the packing material is not so vivid, which makes logical sense -- it's the stuff you quickly want to discard so you can go to playing with the product itself. However, had there been a bright, yellow, plastic, floppy-looking thingy inside the floppy drive, I have little doubt I would have retained it with the Guided Tour and other floppies, as such is ingrained within my personality. And yet, had the insert been a piece of cardboard, I might have thrown it in the original box with the rest of the packing material.
In summary, everything within me cries out to say there never was a yellow, plastic, disk insert in the Macintosh 128k, which leads me to conclude it probably wasn't used on other 400k drives either (whether internal or external). However, I would love to find someone out there who has a "rare" never-unboxed Macintosh 128k so my gut feeling can be confirmed.