I was a spray painter over 30 years ago, and at that time one of the "old timers" told me they used to hand cast metal flakes when they first came out. (At the time they were big flakes, and wouldn't fit through the gun nozzel.) I also like the dry spray idea. However, don't sand right on the flakes. They are typically colored aluminum, and sanding them will expose the base color, and turn it dark. The guy I knew years ago said they used to go over them with a roller to flatten them out, then put on a heavy clear coat before sanding. Even when using modern spray equipment, clear should be sprayed over a metalflake coat to make sure the flakes are completely covered before wetsanding.
Do a test panel and post pictures!
LanceB