Preventing tempo marks stacking

[Richard Sabey]

I have an orchestral piece where the score and all the parts have the same tempo marks at the same moments in musical time. If a tempo mark's text is long, and an orchestral part has little music between it and the next tempo mark, Lilypond will allow the music to take less horizontal space than the earlier tempo mark takes; this forces the later tempo mark to stack above the earlier one.

How can I prevent this, please, by forcing Lilypond to make the music take more horizontal space than the earlier tempo mark, without creating more problems?

I know about \textLengthOn but it is to no avail here.

I saw issue 1024 but its solution doesn't solve this problem. See the second \score block in the example below. It sometimes (but not always - how come?) prevents any notes (stems, anyway) being placed below a tempo mark, and still doesn't prevent tempo marks stacking.

%% begin example %%
\version "2.13.55"

global =
{
    \tempo "Allegro moderato" 4=104
    s1*10
    \tempo "Molto meno mosso" 4=72
    s1*10
    \tempo "Tempo primo"
    s1
}

forEachPart =
<<
    \global
    \compressFullBarRests
>>

music = { c'2 d' R1*9 e'2 f' R1*10 }

\score
{
    {
        \new Staff << \forEachPart \music >>
    }
    \layout { }
} % score

\score
{
    {
        \new Staff
        << \override Score.MetronomeMark #'extra-spacing-width = #'(0 . 0) % cf http://code.google.com/p/lilypond/issues/detail?id=1024
            \forEachPart \music
        >>
    }
    \layout { }
} % score

%% end example %%