Never tried it in soap, actually never thought about it. If you are just looking for color, I guess it will work. Since, jaggery syrup and honey really look similar, its a common adulterant found in honey. So, why not try it in soap instead of honey, it wont give the fragrance though.
I dont know how molasses tastes like, never used it before, not easily available here. But jaggery is really common here, can find at any grocery shop.