I've installed a few humidity control systems in non-player uprights,
and in my experience they help somewhat in stabilizing the tuning
between visits and have postponed at least one re-stringing job.  I've
yet to encounter one in a player piano.

Dammp-Chaser (the most prominent, and perhaps the only manufacturer
of piano-specific systems) markets a model for player pianos that
fits almost entirely in the back of the piano (aptly called the
"Backside System"), with the humidistat control placed inside under
the keybed, the understanding being that over a period of weeks the
entire soundboard will absorb the correct moisture.  If any advertising
claims have been made as to the player action itself; I don't know
about them.  It would be good to hear from anybody on the list who
has installed one of these setups, as to how things have worked out.

As for player grands, I would think most don't have the space to
properly install a system: the humidifier tanks are usually hung
between the beams, with the heater rods spaced below the beams.
While the heat given off is fairly mild, I'd have some concerns about
the effect on the player components in a crowded space with limited
air circulation.

Apropos of that -- while the present systems are controlled by a
"humidistat", the devices first appeared on the market as a simple
heating rod, and the customer was expected to remember to pull out
the plug when heating season came around.  Not surprisingly, plenty
of owners just left the thing switched on all year.  Many rods were
installed in spinets, close to the infamous plastic action elbows
used in the 1950s, and it's believed that the rods cooked the elbows
and really shortened their life-span; just an example of unintended
consequences.

Again, maybe there is an MMDer who can tell us from personal experience
with a grand piano.

If I were remodeling/building a house, or having a music room added,
I'd consult the architect about getting an appropriate humidity control
system built in, as museums and archival libraries do.  Otherwise,
Bruce Clark's plastic tent sounds like a good plan for helping the
piano through a brutal winter.

Richard Friedman
Upstate New York