Dan Wilson pointed out the drawback of strict proportioning of pressure:
at soft volume the Accompaniment notes will fail completely.  My earlier
definition of my goal is better: I want the Accomp intensity suppressed a
constant decibel value lower than the Theme.  3 db is 70% hammer velocity
(and the result of 50% accelerating force), therefore the _working_
Accomp pressure will be 50% of the working Theme pressure.  "Working
pressure" (for the hammer-accelerating force) is defined as the actual
stack pressure minus the pressure needed to overcome the dead weight of
the hammer and damper.  (Or, in a vertical piano, it's mostly the force
of the damper spring.)

Craig is right, the Hickman regulator _will_ do the task, in conjunction
with an adaptation of the Ampico B "voltage divider".  The trick is that
the First Intensity Adjuster is replaced by a fixed reference vacuum of
about 4 inches; this is the effective "dead weight" force.  The reference
regulator would be similar to a pump spill, like on the Ampico B pump.

This scheme nicely solves the problem Dan Wilson raises.  As the
pedaling (Theme) vacuum diminishes the Accompaniment vacuum must also
diminish, but both are proportioned with respect to the 4-inches vacuum
"floor".  Thus the proportioning is referenced about the "dead weight"
force, and so both Theme and Accomp notes will ultimately fail together
as the Theme (pumping) pressure is diminished toward zero.

I'll provide the physics and math discussion privately to anyone who's
interested -- but "the real proof is in the pumping!"

Robbie Rhodes