I have two broad hatchets. One I made a bent handle by utilizing material with a suitable amount of curve to the grain. The other was a Plumb with a factory straight handle. I enjoy both. Quite frankly the only time I feel the need for a bent haft on a broad hatchet is when hewing on large timbers. I guess what I am saying is that you might want to think about if you really need a curved haft.
If you do determine need or just plain want (which is perfectly valid) a curved haft then you might also think about steam bending a straight haft for it. It shouldn't be too difficult to wish up an apparatus to steam a haft of suitable size for a broad hatchet.