centered dynamics

[Werner LEMBERG]

After playing a bit with an extra (middle) staff for dynamics as
demonstrated in piano-dynamics.ly I come to the conclusion that it is
still not the right solution.  Reason for this is that staves are
always stacked.  If, for example, the left-hand staff contains a high
note with a lot of ledger lines, the dynamics staff is shifted up to
the right-hand staff.  At least for my piece, the number of vertical
corrections to be applied manually is about the same as with attaching
dynamics in the traditional way.

I want a mechanism to make dynamics really centered between two
staves, and only vertical collisions should cause them to be moved.
Could a vertical spanner be used for centering the dynamics staff
line?


    Werner