It looks as though I spoke too soon. After applying various of
\override Voice.NoteColumn #'force-hshift
\override NoteColumn #'force-hshift
\shiftOn
in an engraving which needs a good many of horizontal shifts, I found
that sometimes they work and sometimes they don't.
It seems that the behaviour of NoteColumn #'force-hshift varies
considerably with the number and configuration of voices.
Examples:
1. Using voices One, Two, Two: overall alignment is right but
force-hshift does not work.
2. Using voices One, Two, Four: force-hshift now works but the overall
alignment is very unsatisfactory.
3. Using voices One, Two: this is the best-looking of the three (after
the last tie is nudged up a little), but the rest of the piece really
needs three voices.