Since it's just a dressing you may not need anything to substitute, I might try mixing up some of the dressing and tasting it to see if it really needs anything because it might not. I think some brown rice syrup could be a natural fit here if you think it does need something, but it's hard to find in small bottles so if it's not something you think you'll use and/or already own it'd be impractical for the tiny amount you would use for this salad.
Stevia is a good substitute we use for sugar. We have to watch our intake of any type of sugar-- natural or processed. Stevia (I would use a few drops-- VERY SWEET-- of pure liquid Stevia) is a natural sweetner from the Stevia plant. You can find it in health stores. It does not feed yeast so you can't use it in baked goods that require sugar to feed the yeast.