Comment #8 on issue 2989 by [email protected]: Funny alignment of different bar lines at the end of a system
http://code.google.com/p/lilypond/issues/detail?id=2989
If I place the span bar according to your rules, then it would be misplaced in combination with the simple bar line in each lower staff according to the picture in #6. So perhaps something like \once\override BarLine #'span-bar-alignment = #LEFT (which of course doesn't work yet)?
