Hi Dave,
interesting project.
I think if you tilt all 3 motors, vectored yaw should also be possible. Reverse Tricopter-VTOL Plane - #19 by Rolf
We ultimately had to do without vectored yaw because the rear tiltservos always wanted to tip the motors forward after the forward transition. This was not desirable, as we only used the front motor for propulsion.
Rolf