Cool idea but what is so hard about just masking off the windows? If you're gonna do something do it right. If you want your paint to be good you spend hours prepping it. This just seems like a waste of money also that gun isn't special any normal gun can change vertical/horizontal spray. IMO a gun ran with a compressor is more consistent as well.
To each his own though just don't think this is a good idea to paint your car with. It's like something Billy Mays would advertise if he wasn't dead.