So I wrote a program to change each of the 0x5f to 0x41 (and A) and then see what happened.
It turns out I managed to get an underlined A. My fix is to change that to a space 0x20 and leave it at that. I never did find the comparison function. Otherwise it would be trivial to hack an Apple printer to replace one character with another. Since the next bytes were 0 (and Intel has a bad byte ordering), I can replace an _ with most any unicode character in the font.
There is a 95 or 0x5f at about 0x3EBF in the file /usr/libexec/cups/filter/cgtexttopdf. Change that to a 20 to fix the bug or anything else you want. You can get an Omega U+03A9 by putting a A9 where the 5f was and 03 in the next zero byte.
Os X 10.10.5 14F2411 Yosemite Darwin Kernel Version 14.5.0.