I'm wondering if a YKK #10 plastic jacket zipper would work. It's actually made to be heavy duty, but as its plastic the weight isn't bad at all. #10 means the teeth are 10mm wide thus requiring a wider, bulkier tab that should be easier to grasp. They also make a #8 (8mm). If the original zip is a "daintier" metal - due to the light weight fabric of the coat, the additional bulk of a plastic zip should help.