If the motor thrust will be used to provide some lift as well as thrust then the line of thrust must be through the CG of the overall plane. So yeah, you'd want to do a test glider and find out just where the CG is best to be placed and then arrange the motor thrust lines to pass through that point in a manner that keeps the thrust line thru the CG at all points of rotation. So the rotational axis would need to be at or very close to the CG position.

Something else for him to consider is that if the model lifts off in a level attitude but climbing to some extent due to the thrust then the wings are operating at a negative angle of attack. To get the very best short takeoff performance the model would either need to be set up with landing gear that presents the wings to the airflow at a high angle of attack or the wings shoudl be set up to pivot to some extent along with the motors. Otherwise the lift of the wings during the negative angle of attack portion will be fighting the lift of the motors. And without some sort of zero airspeed controlls he won't be stable in taking off vertically. The best he can hope for would be a slow speed liftoff. And again that would imply that the wings should be angled such that the model can either have the wings at a high angle of attack or rotate the fuselage to get the wings to such an angle quickly and as part of the wheels leaving the ground.