Sometimes asking the archer to use their pinky finger to touch their neck actually solves this. When the pinky is pointed towards or slightly touching the neck it rotates the draw hand and sets it in a vertical position.
I agree. This should work provided the wrist isn't already at the limit of rotation, which is the point I was making.
On a slight thought tangent: If you tense either/both thumb and little finger, the working tendons (going through the wrist) that control these digits may introduce a stress through the wrist. I've always taken the line " If you don't need to work a muscle, then don't. " i.e. keep everything as relaxed as possible, especially around the string hand. Same line as "flyaway" fingers (almost) arching back after the loose; tension where none is needed in that case.
At full draw, the orientation of the string hand palm is dependant on the position of its connected elbow relative to the shoulder.
With the elbow in the bow plane (defined by the bow and the string) and the forearm following the DFL (draw force line, also in the bow plane), the elbow will (typically) sit above, and forward of, the shoulder joint: The shoulder isn't in the bow plane therefore the upper arm isn't in the bow plane.
The upper arm and the forearm define a plane and the elbow "hinge pin" is normal (mathematically) to it. Since this plane isn't usually horizontal (unless you draw straight across the shoulders, Henry VIII medieval style), the elbow hinge is unlikely to be vertical.
The string hand palm, near the limit of its rotation, is typically about parallel to the elbow hinge (pin). e.g. Try this on yourself. Bend your elbow, with forearm across at waist height, and set your hand /palm parallel to your upper arm (flat on your belt). Rotate your palm /forearm both ways and check the limits you can achieve without external aid. My limit, palm up and palm down: The flat hand lies parallel to the elbow hinge. Does that make sense? I'd be interested to know if my limit is typical (or if I'm odd , physically. Leave mental out of it?).
For the palm to be truly vertical at full draw, the elbow hinge needs to be near vertical. The plane defined by upper arm isn't horizontal so there's a conflict.
Most folk I see don't have the string hand truly vertical but slightly rotated "palm down".
Thoughts, please, if you have managed to follow that convoluted thought train.