If more salt gets in the motor...just wash it out at the end of the ride. To prevent any damage to the motro from the salt, paint it inside first with that red insulation paint a few people have already used.
So far the only hub I recall reading about here on ES that actually got windings destroyed was one by I think El_Steak that was a motor with internal fins glued on--a fin came off and got stuck in the winding/cover/fins and shredded stuff. Bad Hair Day.
It wouldn't have mattered if his was vented or not.
My 9C has been using the huge-holed covers from the 9C/GM I got from Icecube57 a while back, and even with all the dirt and dust here in Phoenix, and all the metallic road debris, rocks, etc., that abound on our always-under-destruction streets, I've yet to get anything in there. Even dust has been minimal--I get more dust in my house, layered on everything, from one night with the doors and windows open than that motor has gotten in it the whole long time (half a year? more? I forget) the motor's had those covers on it.
Not much dust to see there, right after taking the cover off to swap it to one modified for rear-wheel use: