Well, several threads seem to have coalesced here.
 
I measured the RH in the shop, and it is the same out there as it is in the
house at a given temperature.  This is what you would predict if you were
thinking carefully, since you are starting out in both cases with outside
air at the same absolute humidity.  It seems to me that where you might see
a difference in terms of effects on harpsichords, people, etc., is if you
are exposed directly to the flow from a register in a forced air system -
that air, being warmer than the average temperature in the room, will have
lower relative humidity, hence will tend to dry things out more.
 
One of the advantages touted for radiant heat is that one can feel
comfortable at a lower air temperature.  The body senses the "radiant
temperature" - just like sitting in sun on a cool day with no wind.  I had
assumed that this would somehow apply also to the hpschd, which if at a
lower temperature would tend to dry out less than with a forced air heating
system.
 
However, I think that description is too simplistic.  No matter what heating
system us used, there will exist a variety of temperatures in the room, and
heat will be exchanged by a variety of mechanisms.  A harpsichord, which
presents a flat horizontal surface parallel to the floor, I imagine could
easily arrive, via radiative transfer, at a temperature HIGHER than the
average air temperature in the room, which could lead to more rather than
less drying of the wood. That could explain why an instrument in such
surroundings would be prone to cracking.
 
At a more mundane level, in trying to evaluate the efficacy of my new
heating system, I am having trouble finding a thermometer I can trust.  I
now suspect that the one I started out with reads at least five degrees
high.  I think I can probably trust the ones built into the Radio Shack
gizmo, one each for outside and inside, which agree with each other to
within less than a degree. Using these to read ambient air temperature in
the shop, it feels warmer there than in the house at an air temperature that
is five degrees cooler.
 
FWIW.
 
Jim B.