Okay, so if anyone is intersted, I kludged a work-around that was adequate for this one section, though I'd hate to try to implement it for an entire document. I set up a paragraph style with center-aligned tab stops every x points (I settled on 5.2 after much experimenting). I had to reduce the space width by 20% to force the characters to sit that close together – otherwise they kept overflowing to the next tab stop at only 5.2 pt apart. Without reducing the space width, the tab stops had to be too far apart for a good result. Then I edited my text (in Windows Notepad, because I don't like to use the story editor if I don't have to) to place a tab before every character. If it were a really big block of text, I'd have used some online text tool like textmechanic.com to automate this, but it was easier to just do it manually for this small section.* I think the result looks reasonably good even if it is a kludge and I had to put weird formatting (tabs) into the text to make it work. I'm attaching crops of screenshots from "before" (in the normal font with proportional figures) and "after" (my simulation of tabular figures). The font, if you're curious, is Kingsbridge Semicondensed Book. If anyone knows a better (or another) way, please share!
*Actually, I am lying here. It just didn't cross my mind to use a tool to do this until it was done.