Well the sheath modification has been successfully completed; but unfortunately, my photographic skills aren't at all up to par.
Y'all are going to have to be a little bit creative to try and see what transpired:
This first pic attempts to show the litlle "ridges, bumps, or whatever they are called" inside of the retention tab. If you look really hard towards the upper left and right on the inside of the sheath, you can almost see them.
Using a razor blade, they were removed. Since they were barely visible in the first pic, it may not look a whole lot different.
Next some heat was applied via the heat shrink gun on low setting to try and re-form the retention tab to the desired shape. It took some experimentation to see how much heat was required to get the material soft enough to bend, without turning the sheath into a glob of melted thermoplastic, but eventually I got "brave" enough. A pair of needle-nosed pliers aided the shaping process. It is probably a little bit easier to see that the corners of the retention tab have been folded inward, as well as the whole tab itself.
This is the "finished product", with the knife in place.
Retention is
substantially improved; this thing ain't falling out of its sheath any more! Now to ty-rap the sheath to the shoulder strap of the pack in the typical inverted position, and the project is done.
Hope this helps someone else considering something similar.