I had a hair flap surgery done to replace my hairline in 1985. When I went to India in 2005, I might have been a NW5 except for the dense hairline from the flap. In India, a doctor used body hair to rebuild temple points, make my hairline more natural, and fill in behind the flap. Some of that body hair grew and is still growing. But I believe some has disappeared with age, like it has from the donor areas.
I have quite a lot of documentation, and just realized that it might be useful for this board if I took good, new pictures of these transplanted areas and had a trained specialist post the lot with comment. I will try to get that done sometime this spring.
My point is that body hair, for some, might disappear with age. If you want to use body hair on your scalp, your future plan should include the possibility that it might not be permanent.