overlapping contexts and consequences

[Kieren MacMillan]

Hello all,

In my choral scores, I often want centered dynamics and centered lyrics, e.g.,

  \new ChoirStaff <<
    \new Staff = "SA" \lady_music
    \new Dynamics \choir_dynamics
    \new Lyrics \choir_lyrics
    \new Staff = "TB" \man_music
  >>

1. When the dynamics and lyrics *don't* occupy the same moment (i.e., 
horizontal placement), I would like the lyrics and dynamics to be "squashed" 
onto the same vertical axis.

2. When the dynamics and lyrics *do* occupy the same moment, and there is 
opportunity for the dynamic to slide left or right (e.g., at the beginning of a 
system), I would like that to happen (i.e., *rather* than the dynamic sliding 
up or down).

3. When the dynamics and lyrics *do* occupy the same moment, and there is no 
[reasonable] opportunity for the dynamic to slide left or right (e.g., in the 
middle of a measure), I would like to set a #'direction parameter which would 
determine which way (UP or DOWN) the dynamic would slide to avoid collision.

What is the best way to achieve any or all of these results "automagically"?

I assume it will involve modifying the extra-spacing-height and/or -width of 
both the Lyrics (or LyricText) and Dynamics (or DynamicText or 
DynamicLineSpanner) contexts, but I'm hoping it's no more than that (i.e., that 
it won't require a whole bunch of new programming or complex custom engravers).

Thanks,
Kieren.