center text vertically between staves - indicating keyboard changes in organ music

[Peter Van Kranenburg]

Hello,

How can I center text vercially between the staves of a PianoStaff?

The two way's I can find in the learning manual (using lyrics and 
defining a Dynamics context, section A.2) seem a bit overdone for just a 
few marks. Is there another - more local - way?

Shifting the text manually by changing the Y-offset is not really an 
option because the space between the staves is not fixed.

The reason:
I'm typesetting music for organ. For the keyboard part I use a 
PianoStaff. I want to indicate keyboard changes. E.g.
Both hands on Grand Clavier
Left hand on Grand Clavier, right hand on Positif
Right hand on Positif, left hand on Grand Clavier
Both hands on Positif
Both hands on Echo
etc.

In the cases where both hands go to the same keyboard, I would like to 
put the indication vertically centered between the staves of the PianoStaff.

Thanks in advance.

best regards,
Peter van Kranenburg