Centering markup between staves

[Robert Clausecker]

Hello all!

I'm currently trying to transcribe an organ score [1] which notes register
changes between the two staves for the left and right hand. For æsthetic
reasons, these annotations should be vertically centred precisely between
the two staves.

I've unsuccessfully tried to do this by various means (using a Dynamics
context with the annotations as markup, using a Lyrics context with the
annotations as lyrics [lyrics without a corresponding voice really suck a
lot], directly attached to a notes) and none of these yielded a satisfying
result. The annotations are always more or less off-center. Is there a way
to get the desired result?

Thank you for your help in advance!

Yours,
Robert Clausecker

[1]: http://imslp.org/wiki/Special:ImagefromIndex/82912