Pat,  In principle, I think you could fabricate a valve as you
describe.  The Ampico amplifier switch box does something similar,
with two suction supplies coming to the box from the treble and bass
stacks, both capped by leather flap valves aimed toward the stack,
so that the one with greater suction opens the valve and powers the
amplifier, while the one with less vacuum remains closed.

However, to do that only for a small bleed flow rather than the
somewhat greater flow of the Ampico amplifier power would require
very supple, small leather flaps with good seating.

However, I have faced the same issue on both a Steinway OR and on
a rare but nice XR that has both the hammer lift and keybed shifter.
My preferred solution both mechanically and musically, and what
I believe was probably the original design, is to reverse the functions
you describe.  That is, put the keybed shift triggered by tracker hole
#95 to provide the soft shading coded by the roll, and have the hammer
lift operate only when the soft setting is selected by the control
lever on the keyslip.

This has the musical advantage that the roll then controls the exact
same soft pedal control (the action shift) that the original artist
controlled, which gives a different musical tone because of sympathetic
vibration of the unstruck strings and the change of striking point on
the hammers, rather than just attenuating loudness the way a hammer
lift does.  Since the mechanical balance and lost motion of each key
remains the same, there is no need to add any compensation while hole
#95 is triggering the action shift.

Those who don't like this solution point out that the action shifter
is a little slow to operate and uses quite a bit of vacuum power.  Both
points are true to a degree, but the real soft pedal of any grand is
slower to operate by foot than the damper pedal, and is rarely (if
ever) used for very short, quick effects.  And a well-rebuilt Duo-Art
pump with tight expression pneumatics will have plenty of power to run
the action shifter for the soft pedal coding on any roll I've ever
seen.

That way, the soft setting on the keyslip lever does what it is
supposed to do: the modulator reduces power so that the higher levels
are quieter, the hammers are lifted to provide further softness (but
of an artificial type that a performing artist on a normal piano could
not do) and level two is activated to compensate for both the reduced
vacuum and lost motion introduced by the hammer lift, so that
roll-coded levels 0 and 1 still play audibly.

That way there is still a reasonable facsimile of expression control,
but overall much softer.  And hole #95 still operates the action
shifter, providing the correct shading variations coded for in the
roll, again with no additional compensation needed because the action
mechanics remain unchanged.

Ralph Nielsen
http://www.historicpianos.com/