I think it's a great idea and have thought about doing this with ours.
Why not take advantage of it if the window is properly hinged and won't fall out and it save you a step in a real emergency. LOL!
I seem to recall someone posted a while back that they used a wooden dowel as a holder.
Often thought about making a screen that attached via Velcro.
Check out this link http://www.jeff-z.com/rv/amods/amods.html
About halfway down the page. Pretty nice, simple and clean mod using two 45 degree pvc elbows.