I would be concerned about moisture in the air condensing into the sensor.
I've seen lots of people have the sensor stop working when they attempt to clean the housing, and I had assumed that the reason why Toyota put it on the top like that was to prevent condensate (or indeed dust, though that should not get through the air filter) in the air from getting into the electronics. You might find out after driving on a humid, foggy or rainy day.
The sensor itself certainly isn't dependent on gravity, and so should work perfectly well in any orientation.
Compromise: Tuck the wire out of the way with the sensor on its side with the plug downwards. You could still see the arrow from the fender side...