You could wire up a 1' x 2' tiltable panel to the back of the Zero. It should be good for about 0.3 amps at 85V (~25w) and would weigh about five pounds.
After an hour of charging in direct sunlight, you'd have an extra 20 Wh back in the battery, which would take you about
half a mile at 15-20 mph.
Over an eight hour workday, depending on latitude, you could probably pick up
3-5 miles of range at those low speeds .. 100-160 Wh, or equivalent to about 10-15 minutes of charging from the wall.
Edit: adjusted estimates downward .. 15-20 mph is probably in the range of 40-50 Wh/mile in real traffic .. which gives a ZF9 bike a range of 160-200 milesNote that the extra weight and aerodynamic penalty of a large fixed panel may ultimately end up making the bike less efficient.. on a long ride, you might end up losing more range due to the panel than you gain. Notably, Terry doesn't have those panels wired up to his bike now.
Solar on-bike is a great idea. It's just not super practical in a motorcycle application; you either need far more solar power, or you need a more efficient vehicle, ie an electric bike.
Other threads where this has been discussed:
http://electricmotorcycleforum.com/boards/index.php?topic=2897.0http://electricmotorcycleforum.com/boards/index.php?topic=2206.0