Actaully I am running a partial grille block on my Insight, not any WAI. I also run the tires at sidewall max pressure.
Because most hybrids already runn smaller engines it may not be as beneficial to run a WAI, but if it could be accomplished without being easily reversible. it might be beneficial.
I find even hybrids respond very well to hypermiling techniques, particularly driving at lower speeds, and anticipating traffic lights. You can also do mini pulse and glides, especially if you have slight hills where you can coast downhill at max mpg while keeping your speed close to constant.
Very slight throttle position changes help, as well as moderate (think about 40%) acceleration form a stop. Get up to speed and then keep the throttle position changes as small as practical.