why couldn't they have filmed the scene when she had her long hair? Or given her a wig?
I believe that they kept the short haircut because that's the image of Fantine that Valjean had most prevalent in his mind. He saw her only briefly with long hair, and even then she had it covered. When the two interacted meaningfully, she had short hair. Since Fantine isn't literally appearing as an angel, but is Valjean's projection, it makes sense that she'd look in death to him the way she looked in life. Personally, I like it.