I'm kind of surprised, what with all the retro computing emulation software and skills I see getting posted here, that we don't have a Voyager emulator scene and a group of people standing by to run scenarios and propose fixes.
Aside from the lack of schematics or listings, there was the problem of the assembler being incomplete!
"One problem Lander software developers had was that no adequate assembler was ever written for the computer, perhaps because of the changing nature of the instruction set.(110) Patches had to be hand-coded in octal, with many jumps to unused memory space because of the lack of an assembler with relocatable addressing." p.174 on Viking, which used the same computer
[1]https://ntrs.nasa.gov/api/citations/19880069935/downloads/19... (page 174 onwards)
Though given the apparent level of NASA involvement in the 402's design, and the lack of evidence I can find for use outside of NASA, it might as well be called custom.