I believe this can be achieved by laying a pad of felt on the strings so that the hammers hit the soft pad of felt. One would still need to be careful not to strike the keys too hard.

Any time the hammers hit the strings there will be some sound.
On an audio reinforced stage, that could be cumbersome. Blocking
the hammers could pose a long term $ problem for the piano. If it is
an instrument of critical use, the overseer of the instrument will not
like any of it.
I am a piano technician that services such instruments and the
university staff gets pretty uptight about even the uncovering of the piano
by people not on the keyboard staff. If you can muster a technician, a soft baby
blanket gaffed to the rim just under the plate (strings) will allow some
travel and maybe even letoff. Designs vary but under all circumstances be
careful of the dampers. Batting stuffed between the action and the strings
might work without threatening the action but it would take some skill/experience
to get the action out of the piano to do that. . . I removed the hammers the
last time it came up. They needed to be filed anyway but I wouldn't
recommend that solution even though a well balanced action will look great
as your fake Farante runs over the keys. A friendly technician can also remove
the upper action and leave the keyboard with just a strip of elastic stretched
over the back ends of the keys. In theory that would work but I would not
do it to one of the above mentioned fine instruments. If all else fails,
cast apiano player! Good luck!