I heard recently (in Mur Lafferty's podcast "I Should Be Writing") of some people doing physiological response studies on movie audiences, finding that they breathe and blink all at once at certain times. Mur then speculated that the same kinds of things might happen for readers of novels, but that it would happen at their own pace, so it might be hard to do such a study.
You know what might work, though? If you're reading a novel on a tablet, there's a camera pointed right at you that could, in theory, do some eye-tracking and other measurements. If you were looking to do a very broad study of novel reading physiological responses at a low cost, I would look at that. Consent would be hard to get, though, and the data would vary quite wildly, I imagine. Still, it's a place to start, right?
Mokalus of Borg
PS - That's assuming the data is worth trying to gather at all.
PPS - I can't imagine what it would be used for.