What would I do?  Faced with the same problems in my own home, I do
not encourage using electrical piano "toasting" or "steaming" devices
to add or subtract moisture on the interior of a piano and not on the
exterior of the piano.

The reason: if you add moisture to one side of a piece of wood and
not the other, the result is stress and warping of the wood.  But if
the same amount of moisture or dryness is applied to both sides of
the wood it will remain stable and have much less stress.

In my opinion, the best solution would be to make an attempt at
climatizing the room in which the piano is located.  This can be
accomplished by keeping all doors and windows to the room closed,
using air conditioning in the summer, plus heat if it gets too cold,
and using a room humidifier during the very dry months when artificial
indoor heat is required.

In spite of making the best attempts in using air conditioning and
humidifiers, one cannot precisely regulate the exact percentage of
humidity within a room, but one can reduce the extremes. In my case,
I can increase the humidity to 35% in winter and reduce it in summer
to 50%.  This is far better than 5% in winter and 95% in summer.

Another concern is glue joints.  Keeping the humidity relatively stable
in a room will protect glue joints, too.

It is my belief that Phenoseal would eventually cause more problems
than it is worth.  I am reminded of this from experiences in the boat
industry where a few years ago it became a fad to fiberglass interior
and exterior of wooden boats.  In theory it made sense, but the
fiberglass coating did not cover 100% of the wood, and moisture
eventually became trapped between the layers, causing the wood to rot
and make matters worse.

Bruce Clark