True, thank you--I've incorporated your suggestion below.
Thinking about this some more, I figure the next logical step is to write an engraver which allows you to specify the number of measures for each line of the score. I've come up with the solution below.
If you use the (commented-out) override and you have measures left over, these will be crammed into the last line. Without the 'line-break-permission override, Lily will space the remaining measures as usual.
\version "2.15.38"
#(define (custom-line-breaks-engraver bar-list)
(let ((total (1+ (car bar-list)))) ;; not sure why increment necessary
(lambda (context)
(make-engraver
(acknowledgers ((paper-column-interface engraver grob source-engraver)
(let ((current-bar (ly:context-property context 'currentBarNumber)))
(if (and (pair? bar-list)
(= (remainder current-bar total) 0)
(eq? #t (ly:grob-property grob 'non-musical)))
(begin
(set! (ly:grob-property grob 'line-break-permission) 'force)
(if (not (null? (cdr bar-list)))
(begin
(set! bar-list (cdr bar-list))
(set! total (+ total (car bar-list))))
(set! bar-list '())))))))))))
\relative c' {
\repeat unfold 10 {
\time 2/4
c4 c
\time 4/4
c c c c
\time 3/4
c c c
\time 1/4
c
\time 13/4
c c c c c c c c c c c c c
\time 9/4
c c c c c c c c c
\time 2/4
c c
\time 7/8
c8 c c c c c c
\time 2/2
c2 c
}
}
\layout {
\context {
\Score
%% The following line necessary if you want to put more
%% measures to a line than Lily wants to allow.
%% Remaining measures will be crammed into last line.
%\override NonMusicalPaperColumn #'line-break-permission = ##f
\consists #(custom-line-breaks-engraver '(1 2 3 4 5 1 2 3 4 5))
}