since akoyas come out perfectly round, and consist of little more than a thin layer of nacre over a round nucleus, why don't pearl farms use round nuclei to produce round fw? it seems to me perfectly round fw would be a lucrative market segment, combining the shape of the akoyas with a much lower price.

Freshwater pearls are nucleated with beads as well, this is just not the typical fashion. See the thread here:
http://www.pearl-guide.com/forum/showthread.php?t=27

There is also a thread somewhere in the forum where JShepherd described the look of nucleated fwp, but I was unable to pull it through a short perusal.

I think if more farms start to nucleate, and they are able to perfect the technique, this will probably be the fashion of things to come for large freshwater pearls. It has benefits such as shorter duration (culturing cycle), and the ability to produce larger, more round pearls. Unfortunately, from what I understand, the technology is not quite there yet to produce them consistently without the 'tail'. This may be because the nucleation goes into the mantle instead of the gonad.