This time not a question, but an answer...
I needed a style, a font, with spaces made visible, couldn't find it,
modified a font.
If you need it some day, it lives at http://media.nedcad.nl/support/
A few thing were interesting to learn:
Copying from PDF-readers: Trailing space character(s) of a selection are
skipped.
In Writer, formatting marks (Ctrl-F10) itself also contain spaces that
are made visible with this font.
Best regards, Wiebe
(background information:
https://nedcad.nl/en/paste-on-the-command-line/)