I don't think I would go too heavy with dark brown sugar (maybe a little wouldn't hurt), but perhaps if you end up reducing some of the Special B in your recipe to compensate/offset for the increased molasses flavor, it might work out...I personally would simply use table sugar, if pressed, between the two. I guess it also depends what other malts you have going into this batch.
I just think that my attempts with molasses (straight up) have not been satisfactory. YMMV, of course.