So the good news: The replacement impellers do spin very easily. The lightest breeze your mouth can produce will spin them. Perhaps they are even too sensitive, as just moving the impeller will make the blades move.
The surprise: They are really small - from left to right is the case fan, the impeller, and a quarter:
My Radio Shack IR emitter/detector pair were going to be too big & bright for the job, so I went to Fry's and picked up an NTE3029B IR emitter and the matched NTE3034A phototransistor. These devices are small, side-looking devices with integrated lenses and are much more appropriate for the job with the small impeller.
Here is the Kestrel impeller as a "breath input" using an Arduino and a case fan "breath output":
According to the Nielsen-Kellerman patent, there actually is a little magnet in the impeller somewhere which might be able to sensed with the proper sensor. I am considering getting a Hall Effect sensor and trying it out, but at least I know the IR scheme works.