How can experimental compliance (willingness of participants to participate) be measured? I would like to have a measure for how willing and eager participants were during an experiment. I have the intuition that their eagerness to comply will profoundly alter results of psychological and even psychophysiological experiments, but I lack a good way of experimentally quantifying this.

As a seed, one way would simply be to ask them following the experiment, another may be to include trick questions with an obvious but incorrect response - this would however tend to conflate willingness with attention.

Any suggestions very welcome.
You might look for examples in the Witt, Donnellan &amp; Orlando (2011). They tested subject motivation in a psychology subject pool to see how it varied among groups. There may be follow up studies. Looking through similar studies will give you an idea of the kinds of things people use to assess. This is not a standardized line of research (yet).

Witt, E. A., Donnellan, M. B., &amp; Orlando, M. J. (2011). Timing and selection effects within a psychology subject pool: Personality and sex matter. Personality and Individual Differences, 50, 355-359. doi: 10.1016/j.paid.2010.10.019