I don't know, it seems pretty solid, maybe we can force it with minor damage.
How about removing whole side mirror, covering some area with masking tape, and start painting. I think it's more feasible.
Close the mirror and insert a pry tool into a small gap between cover and black pannel just pry it open spray them and then push it back in place i recomend using some double sided tape to reduce vibration and a rattling sound