Mark,
I've had pretty good luck using a dusting of garden sulfur on dahlias to decrease the leafminer damage, but have never tested its effectiveness on tomatoes. Unfortunately, if sulfur is used during hot weather, it can cause almost as much damage as the leafminers. Leafminers are difficult to control with any pesticide - even the systemic pesticides require frequent application, and you wouldn't want to use systemics on food plants anyway. You might try a light dusting of sulfur on the lower portion of the plant (on a couple of plants) as a test.
Please don't use Sevin unless it is absolutely necessary. As mentioned above, it is
not a safe pesticide, and will not be of much, if any, help for leafminer control.
Besides being deadly to bees, if you apply Sevin frequently you are at a much higher risk of getting deadly skin cancer. The tip of the iceberg:
http://www.environmentalhealthnews.o...ly-skin-cancer As a commercial farmer I would never consider using Sevin, and can't believe it is still approved for use by the general public.
Steve