Hybrid PX4 + CC3D flight control

So I got bored and as per usual, came up with an idea. Most of the PX4 flight controllers run $100 or more. I can get a Rpi zero w and an openpilot-family flight controller (like a CC3D) for next to nothing. So I started wondering what it would take to deploy the more advanced components of PX4 (like mission planning and execution) on the Rpi and then use the low level stabilization/control components on the CC3D to keep the board level and flying. I think this should be very possible based on the existence of things like the Rpi+navio2 target.

So my questions. Is this actually a reasonable project?

I know how to interface with the CC3D over Uavtalk. I just don’t know where I need to be in the PX4 source tree. Is this a new module I need to create? A new driver? A new target?