The science seems fine...but where is such a regular, rhythmic, change in humidity to be found apart from day-night variations? Perhaps in Metro stations with a sudden rush of air as trains arrive? Does anyone have any more ideas?

Nature finds a way

I guess 2 fans, one blowing low humidity air across the sensor and one blowing high humidity air across it. Just switch each fan on in turn. Shouldn't take too much of the energy produced to keep it going.

Perhaps a mister could provide a humid punch to an airstream. Overcoming mechanical losses may be a challenge, but a solar chimney might eliminate the need for a fan.

There are lots of things that change with humidity or moisture. The forces involved can be enough to crack foundations, for example. That's why people water their yard around the foundation.
I guess the trick is to find something that responds at about the frequency of the humidity change cycle.

right wasting water to make power
wasting most of the evaporation energy
sounds like it;s about .001% as good as anything available, plus the bonus danger of mutant bacteria

There may be a lot of power here, but it would be pretty darned diffuse. There are lots of great sources of diffuse power but harvesting any of them on a meaningful scale has always remained elusive. This is entertaining, but I can't imagine it ever being more than a parlor trick. Maybe David Copperfield would pay something for this kind of mechanism to use in one of his acts.

"Solar and wind energy fluctuate dramatically when the sun doesn't shine or the wind doesn't blow, and we have no good way of storing enough of it to supply the grid for long," said Wyss Institute Founding Director Don Ingber, M.D., Ph.D
Once again, a person who ought to know better, assumes that solar power must be limited to collection and conversion on the planetary surface, when only a few thousand miles over their head, the sun shines continuously, without interruption…
Why do even scientists who know better, assume that all problems must be solved using only those things inside the box we call Earth?
We are NOT limited to resources on or in the Earth. We have access to the entire Solar System, and near infinite energy flows by just above our heads. Not only that, but we have the tech to recover it easily!
The Earth and all it's resources are but a minute grain of dust when compared with what the Solar System holds.
To stop thinking inside the box, quit making boxes before you solve the problems!
There is no 'box' until you create it.

coastal valleys have variable humidity sometimes on a scale of minutes. Dry places where fog comes in , like Peru/Chile, Australia, and the African & mediterranean coasts, could also use this to power desalinization. Imagine a bank of spore driven pistons that pressurize reverse osmosis.

SuperFool: Costal mist is desalinated. It can be collected easily, no energy needed. But now it can be used to fuel a generator thanks to Wyss Institute.