The slab elevation is too low according to the local building codes to put any plumbing. As a result, I could raise the slab up and make very steep driveways, or just raise the bathroom and put a few steps up to it. The ceiling at that level is to be 12 feet, so there is plenty of room to come up for the bathroom.
It turns out that it will be cheaper to make the space under the front porch another room in the shop than it would be to fill it with dirt. That's great news—I'm very pleased about gaining the extra space in the shop!