Hi Franklin,
It's nice to see someone in your position show interest providing aero mods to the ecomodder market. As Tom pointed out, please don't forget us non-hybrid drivers. I'd love to buy a real aero body kit from FiberEffects.com.
On potential products: Besides the rear-wheel skirts and back-end mods, I think good side skirts, air dams, tire deflectors and tire boat tails would also be well received by ecomodders.
On the Grill block: How about using the KISS method, making the grill block solid, and letting the cooling fan get air through louvers in the chin tray? Mechanical parts wear and *will* fail. Then, you've got to add controls... All this increases the costs of your product and future maintenance. If my customers were really clamoring for an adjustable opening, I'd look at a bi-metal spring or air pressure, as Different and bzip suggested, to control the damper. Otherwise, KISS and inexpensive.
I hope you do well on HybridEffects.com