Ragged right for contents but not staff?

[Matthew Collett]

With ragged-right (or ragged-last) turned on, music and the staff stop wherever 
is natural.  With it off, both stretch to the end of the line.  Is there any 
straightforward way to stop the music at its natural length, but have empty 
staff lines stretch the full width?

Context:  I'm trying to format Gregorian chant in a style close to that of the 
current Graduale Romanum or the older Liber Usualis.  Both of these behave in 
the way described on the last line of a piece.

My hacked solution:  Turn ragged-right and ragged-last off, turn 
\RemoveEmptyStaves on, and follow the real end of the piece with '\repeat 
unfold 10 { s \bar"" }'.  This works (see attached example), but (a) the 
optimal number of repeats depends on the linewidth and (b) Lilypond (2.14.1) 
grumbles about 'programming error: system with empty extent'.  My instinctive 
reaction to the complaint is "Well, yes.  I told you to remove empty staves, 
and you have, so what's the problem?", but it would obviously be better to have 
a clean compile.

Asperges III.pdf
Description: Adobe PDF document

Best wishes,
Matthew