The humerus is not involved in pronation.
Two problems here:
a) Starting a resistance movement in the "maximally stretched position" is not necessarily a good thing.
b) Subjecting the shoulder joint to a load when it is hyperadducted risks damage to the joint
I would be very cautious about initiating delt work in the position you've described.
In fact, the opposite is true. The first 15-30 degrees of abduction are performed by the supraspinatus muscle, not the deltoid. Therefore, incline-bench lateral raises are largely a supraspinatus exercise. Lean-aways (ie, the opposite of incline laterals) probably have a greater claim to the title 'strictest medial delt exercise.'
Check out this article; it's an oldie but a goodie: