This is an old, dead thread, but I feel I should Add I was in a similair situation. I took 3/4" plywood and cut to size the fit the frame of my window from the outside. Drilled a hole in it big enought to add a wall exhaust fan (box looking ones that point down). Painted the wood to not stick out as much. Then on the inside took out the glass, cut out the screen. Hooked my venting up and stuffed with insulation then put another piece (with a hole as well) on the inside to make like a sandwich. That is probably not all necessry.
The window is on the ground as well comming out of the basement. luckily it comes out right next to the A/C so it doesn't look too inconspicous.
Either way Im sure you've figured it out by now.