Would a slotted "wing" work?
No, it will not work in the way you plotted it.

First, the corners of the duct should be nicely rounded. With the sharp corners you will get a lot of vorticity, but little lift increase. Actually, lift will drop and drag will go up.

Next, a gap works best when connected with a contour break, such as that of a slat or a flap. If you round the corners and add a hinge in order to deflect the rear part of the airfoil, then yes, the gap will help to delay flow separation.