You need a pre-chapter-number chapter label for that (\cl Chapter
). You can set that to e.g. a zero width non-breaking space.
How about this sort of thing in your changes.txt:
at GEN "(\\c 1)$" > "\\cl \ZWNBSP\n\1"
This is a crude and ugly hack, because it will destroy any real \cl
in your input. This is, I think, one of the things that “Use Special features for these books” does, but I think they do some other things too.