Controlling slur shape across line breaks

[Ted Hopp]

[I tried posting this with an attached pdf, but that didn't seem to go through.]
Is there a way to control the shape of a (phrasing) slur on each side
of a line break? The following example generates some awful output:

\version "2.12.1"

\paper {
  indent = #0
  ragged-right = ##t
  ragged-bottom = ##t
}
upper = \relative c' {
 \clef "treble"
 \phrasingSlurUp
 \partial 16
 d16\( | \break
 e4 f g a |
 \time 6/4
 \change Staff = "lower" \stemUp g4 f c1 \) \change Staff = "upper"
\stemNeutral |
 \time 4/4
 d4\( f <a, c e f>2 ~ | \break
 <a c e f>1 \)
}

lower = \relative c {
 \clef "bass"
 \partial 16 r16 | R1 | \time 6/4 R1. \time 4/4 R1*2
}

\score {
 \new PianoStaff <<
   \new Staff = "upper" \upper
   \new Staff = "lower" \lower
 >>
 \layout {}
}

As can be seen, both ends of the slur before the first line break
needs to be raised. After the break, the start of the slur
continuation (in measure 1) is good but the end (in measure 2) needs
to be lowered (a lot!) and, once that is done, there needs to be a lot
more curvature to avoid collision with the notes and the time change.
A slightly different problem can be seen in measure 4, where the slur
ends well but should start the measure much higher up.

I need a way to fix problems like these by controlling the shape of
slurs, and not, for instance, by changing to phrasingSlurDown (which
doesn't work in the context in which these problems first showed up).