Seems like it should work, and I've seen a couple vans with these.

I've found that, unless it's a really heavy, blowing rain, I can leave
the sliders open to the first notch and get none, or very little, coming
in.
When I lived in a very rainy climate (Oregon Willamette Valley) I left
them open like this all year. Now I live where it snows, and found that
melting snow runs inside, so they're shut.

Jose R. Moscoso wrote:
>
> Fellows and fellaws:
> During the past weekend, my family when kayaking into a phosphorescent bay
> (about 20 people, but that another story).  One of my nephews have a toyota
> double cab p'up which have some rain deflectors in the rear windows.  My
> question is: has someone tried this things in the side sliding window
> (obviously, shifting them from horizontal to vertical position).  I suppose
> this will allow to open the windows even during rain without having water
> entering the van.  Anyone BTDT???