I like your idea as that is entirley possible with some of hte 3D acrobatic planes. And is is thecnicaly possible but its something that is not currently availabe with APM plane code. I know that the recent ardupane release has something called quadplane support, which is designed for hybrid VTOL planes so you might want to check that out.
Are you planning to build a custom vehicle or just use a stunt plane with your pixhawk?