XPM probably wouldn't work for my purposes, and I am Not Inclined to learn C, which is thanks — at least in part — to Kate.
But it does look like one could generate/edit XPMs pretty easily with sed, and that seems like a plus.
Vim also has ridiculously overkill syntax highlighting for them. Here's an example via the :TOhtml feature:
https://paste.xinu.at/S6OjH0
Can select the text to see the characters. You'd need a font with a square grid of characters to avoid having it stretched.