Non-printing score-wide dynamics

[David Sumbler]

The first 2 bars of a score are as follows:

invP = \tweak stencil ##f \p

\parallelMusic #'(mid Vone Vtwo Va Vc) {
% bar 0
        \tempo 4 = 152 \partial 2 r2 |
        \tempo "Allegro non troppo" 4 = 152 \partial 2 r2 |
        \tempo "Allegro non troppo" 4 = 152 \partial 2 r2 |
        \tempo "Allegro non troppo" 4 = 152 \partial 2 r8 a\f a a |
        \tempo "Allegro non troppo" 4 = 152 \partial 2 r2 |
% bar 1
        r1 |
        r2 r8 a\f a a |
        r8 a\f a a b4-> a |
        bf4-> a g8 f e d |
        d8\f a' d f a f d a\invP |

The "mid" item is a dummy staff (there is no actual staff) which I use
for tweaking tempi for the midi output: using this I can simulate rits,
fermatas etc.  For the printed score and parts it is not referenced at
all.

Also in bar 1 I have \invP, which as can be seen produces an invisible
'piano' marking: this is simply to avoid the articulate script's
annoying warnings about ambiguous dynamics (which would be less annoying
if they told you exactly where in the music the problem arose!)

However, I have another dynamic problem which it would be nice to get
around.  Later in the music there is a passage marked "dim."; in
addition, each bar has hairpins < and >.  The interpretation of these up
and down dynamics within a more general diminuendo is easy for
musicians, but understandably opaque for articulate.ly.

I can get rid of the warnings about this by putting the "dim." into the
parts as a markup, rather than as a dynamic.  But, although it is not of
vital importance, I wondered if there is some way I could mimic the
effect of the diminuendo (perhaps by using \f, \mf, \mp etc.
successively) in much the same way as I can mimic a rallentando by using
successive \tempo markings in the 'mid' context.

Actually, I am not really sure what kind of context 'mid' is anyway.  I
do not define it anywhere else: I simply include it with the four actual
staves (which are defined with \new Staff etc.) in the \score block
which precedes the \midi command.

How might I add dynamics to 'mid' which would affect all the voices, in
the same way as \tempo changes do?

David