Another idea: We could have a command like partialDuring or partialWith.
Here's what I wrote, renaming Aaron's function:
pickupNotes =
#(define-music-function (mus) (ly:music?)
(_i "Make a partial measure.")
(let* ((mom (ly:music-length mus))
(dur (make-duration-of-length mom)))
(make-music 'SequentialMusic
'elements
(list (context-spec-music
(make-music 'PartialSet
'origin (*location*)
'duration dur)
'Timing)
mus))))
\relative
{
\time 4/4
\pickupNotes {c'8 d} |
e4 f g a
}
I think it's nicely descriptive and doesn't need to use convert-ly.
Carl