een done. S-72 and X-50 prototypes. Its very unstable. The bring a rotor to a controlled stop thing is easy, existing rotor brakes can be geared to align it fairly precisely when it comes to a stop. The lift transition is the issue. It's not just that lift is basically 0. It's that one half of the rotor disc (the theoretical abstract describing the lift forces) has to completely reverse the airflow of the lifting surface.
Its essentially an expanded case of the Retreating Blade Stall problem.
But the retreating half of the rotor disc has to, as some point, go from generating lift from a retreating motion through the air (moving backwards relative to airframe, due to rotation) to generating lift from an advancing motion through the air (moving forwards, relative to airframem though no longer rotating). The easiest way to think about it isnt to think of it as going from rotating to fixed, but rather think about a rotor that is simply being reversed in direction (simplfies a lot of math).
So at some point in the middle there, half the rotor disc will fall below stall speed, and experience a stall similar to the effect of a Retreating Blade Stall. Worse, won't regain sufficient lift until its now going ~100 KIAS in the opposite direction. Think of it as stalling between -100 and +100 KIAS (example number) as it crosses the transition.
The only craft I can see being able to cross that boundary zone would be a very small, very lightweight rotor that is able to make extremely fast accelerations, and thus cross the zone before it's able to affect the craft much. A full scale craft would simply have too much inertia/momentum to be able to make the transition fast enough, without tearing itself to pieces. Likewise for any craft trying to stop the rotor and use forward motion to generate the lift.
Theoretically couldn't it work if the craft had enough sufficient fixed wings to provide most of the lift necessary for level flight at transition speed?
Then you should be able to trim the rotor disk to near zero lift (beginning a relatively mild decent) before braking it to a stop. Once stopped, retrim it for forward lift and level off.
Mind you those big wings would likely do ugly thing to the airflow in vertical lift mode...