Hi Mark, thanks for the comment!
I am specifically running this from the command line, so I can adjust a font source in the build process, rather than directly editing the source (it’s designed by someone else, and I’m hoping to preserve the original as much as possible, and also trying to prevent manual edits from needing to happen more than once).
Unlike adjust the masters (original thread), however, I could manually add this parameter in the font info in my current case (assuming that’s what you meant by “usually I would do the settings before adding it to the font.”).
However, it seems likely that I’ll want to remotely add and set font custom parameters in the future, so I would still be happy to figure this one out.