That works almost fine (thanks), but, using NonMusicalPaperColumn, the "//" symbols are inserted on unnecessary places of the score (top of the page , page break , etc...).
So, I have tried to insert the symbols "by hand" using \mark instead NonMusicalPaperColumn, but I get the symbols right aligned only (\mark on last bar of the system) [ yes, Mats, I did \property Score.RehearsalMark \override #'break-visibility =
#begin-of-line-invisible ] ; that leads me to a new question:
How can I get symbols, texts, etc over the left edge of a system (before clefs, time signs, etc) ?