Using aluminum to mold drywall mud
The drywall mud is just as likely to stick to the aluminum flashing as it is to the board. Just observe how well dried mud sticks to your drywall mud taping knives. It may be possible to put a light coat of mineral oil or silicon lubricant spray on the metal beforehand to minimize the mud adhesion. But you would definitely want to experiment with some small samples before committing to the whole corner. 

I believe that you would be way better off trimming out that edge with an appropriate style of drywall edging and then applying the drywall mud up to and over the fin of the edging.



(Image borrowed from ClarkDietrich)

The reason for this is due to the fact that if you try to build up a large thickness of drywall mud it will always be subject to damage and crumbling out when it gets hit from the corner. 

Stability of the L-Stop Bead edge will likely require that you install a backer strip of wood along the edge of the corner to fill in the space.