Another thought I had was a full-cheek snaffle. Because it would put some pressure on the sides of her face too, which is good for turning. But I think I like the idea of a sidepull even better, since she is used to the indian bosal. But perhaps if you had a access to a full-cheek snaffle, that would be something else to try.
I have owned 3 grays and (knock on wood) haven't seen a melanoma yet. BUT, I did own a wonderful Paint gelding that lived to the age of 33, and he had cancer in his sheath and I believe the inside of one eyelid as well.
Stuff can happen with all our horses. I had a friend who almost lost her 3 yr old QH and the only thing that saved him was colic surgery. So you are not safe even with a "normal" colored horse.
Personally, I love my grays and wouldn't let the thought of melanomas deter me from owning one. But it might discourage me if the horse already had it, just because I wouldn't want the heart-ache of loosing them prematurely. One of the things I did when I went to purchase my current gray Foxtrotter mare was check under her tail for signs of anything abnormal looking! Haven't spotted anything yet.....knock on wood.....