John, one of the main troubles with the hemp you buy nowadays is that the diameter isn't consistant-you only have the strength of the smallest point in the string. Hemp was used for bowstrings in Europe a lot at one point. If you had the raw fibers to twist your own, you could probably make a pretty good string. Flax (linen) makes a good string and is natural, but I don't really consider anything I buy on a spool from a factory to be natural. If I'm gonna buy it on a spool, I'll just get B-50.
Dogbane is a strong fiber and has been used for bowstrings, but if you want a primitive natural string, sinew or rawhide are stronger and more durable than most plant fibers. Most of the existing old Indian bows were strung with sinew, gut, or rawhide. Very few plant fiber strings.