I would agree to Mark Kinsler's comment on all places except his first
word, where I would replace 'Yes' with 'No'.

When things work as they should, then the pressure difference developed
in a pianola pumping system is defined by the tensioning spring and lid
area of the reservoir or regulator.  The same holds for the overpressure
feeding of, e.g., an organ.  These entities are completely independent
of ambient air pressure and altitude, so there really should be no
problem playing the pianola at high altitudes.

Still, there must be a snag somewhere.  This is the one of volume flow
capacity.  The volume displaced per time unit by the pumper bellows
must be adequate, and here the demand becomes bigger at high altitude.

Let us use, for an example, an instrument that needs 1 foot water
column, or roughly 0.4 pounds per square inch, differential pressure.

Boyle's law (Robert Boyle, 1627-91) states that Pressure times Volume
is constant in a specific gas content at isothermal conditions.  So
to reduce pressure in a closed volume of 1 cubic foot from sea level
14.7 PSI into 0.4 PSI less demands a volume increasing to
  14.7/(14.7-0.4)=1.0279 cu ft, an increase of about 2.8%.

At high altitude, e.g., 10 PSI [absolute], you must expand the volume
more, 10/(10-0.4)=1.0417, about 4.2%.

So at the high altitude a bellows suction pump, driven at the same
speed, would produce one or two percent less volume flow of rarified
air, and this should be of very little concern.

Johan Liljencrants
Stockholm, Sweden 
http://www.fonema.se/ 

 [ Johan is moderator of the MMD Pipes Forum.  -- Robbie