Hi,  My Duo-Art grand has a "Normal/Soft" lever in the spool box.
It activates a pneumatic that throws the hammers closer to the strings,
a function that is adjustable.  For 88-note rolls, everything plays
with appropriate intensity for such a roll, but at a softer volume.

If your Duo-Art has no such function, it would be much easier to
fabricate and install one [a lifting hammer rest rail] than to design
an auxiliary reservoir, which I suspect would do nothing for a system
set for quiet playing while at the same time trying to deliver the
William Tell Overture -- it will inevitably run out of air during the
"storm" passage!

The problem is quieting a quarter-ton-plus musical instrument so it
doesn't get in the way of dinner conversation.  It's like convincing
a Bengal tiger to play with a little ball of yarn.

Tim Gautreaux