Yep, I only do this RC stuff for fun and challenge. Making a business out of it would reduce the fun for me.
If I were to make a VTOL product, I would probably go the "strap on" route. The quad would strap to the bottom of the Bixler ready to fly. The only mod to the Bixler airframe would be to make a hole in the bottom of the fuselage to pass the power and servo cables through. I'd keep the battery in the Bixler.
I don't think you want/need any I, just tune P & D. In fact you don't need much if any P & D in the Yaw direction. Check out this quad I built with a fuse. I wanted to see if I could make a Quad fly 3 channel in forward flight...no rudder needed....it worked.
With yaw gyro turned off; or way down; on the Bixler VTOL, the plane will weather vane into the wind when in a steady hover beause for the tail fin.