I take your point Douglass. I guess it depends what the outcome is you are hoping for. I would suggest that in any wood, even if the change of humidity is bound to occur, you still want it to occur as slowly as possible in order to minimise the likelihood of a crack due to rapid changes. That is why we coat the ends of timber to protect the end grain from rapid loss of moisture. We know it will lose the moisture some day, but hope to slow that change and reduce the checks.
Does coating the insides slow that enough to make a difference? I don't honestly know. I suspect it depends on a whole bunch of factors including the type of finish. I would bet it takes a long time for wood to lose moisture if it is encased in melamine, more than a few hours. My sea kayak is encased in epoxy and gel-coat - moisture does not get in, no matter how long it says in the water, so it is theoretically possible with some finishes at some thickness to almost eradicate that transfer of moisture. It would be easy to test on wood if someone had the time and inclination, just coat all sides with different finishes at different thicknesses of some test pieces after ensuring they have the same moisture content and then seeing how quickly it falls in a low humidity environment. I bet it does vary between different finishes and of different thicknesses.
But as you say - that may be a trade off in terms of acoustic properties. Drastically? I'm not so sure. I know tradition says yes, but i'm not sure blind tests would agree. Stephens research in the Savart on the effect of finishes on the vibration properties of soundboards covered in different finishes shows that they all actually increased the Q factor. There are obviously upper limits to the thickness of a finish that you can't cross if you want good sound, but I'm not sure what that is, depending on which finish is used. Classical tradition says Shellac is best, particularly emphasised by luthiers - but then we would, as it separates us from the factories.
What % loss of acoustics is deemed acceptable if it stops you having to repair your guitars all the time? If you coat everything with a finish that means it takes you twice as long to affect repairs, is that a problem if you only need repairs a third of the time? I don't know - 5% humidity is just off the chart to me, so I would imaging it would probably take some quite drastic approach to interior finishing (and exterior) to make any difference.