I did more with SIXELs while at USPS too; one of my pet projects was a Peritek monitor driver/ACP combination that enabled me to use the monochrome 512x512-pixel monitor for both text and SIXEL graphics (ours didn't have the optional character generation ROM).
And the first prototype of the 3547 Automation
Project conversion code from FAX format to SIXEL graphics was developed
in MACRO-11 on the PDP-11 running RSX-11M. It was eventually ported to
A86 on a DOS PC, and the output format changed to PCL5.