As NMK says, putting a fan at the intake grill would create a low pressure area that would increase the intake of dusty air, right where you don't want it.
Right behind the between seats intake grill on the 800 ought to have connected directly to the engine air intake, but that connection was never completed for whatever reason and they just stuck a filter sock on the end of the intake plenum.
For windshield defogging I'd just mount a 12 volt fan directed at the windshield. The airflow will keep the windshield clear of fog.