Elayne Angel performs a navel piercing on a BBW. She explains how she determines the optimal position and shows that not all pierced bellies are flat and tanned. When the woman is standing, this appears to be a "traditional" navel piercing, but when she is reclining, the tissue changes position.
From The Piercing Bible:
Navel Piercing: Weight and Weight Loss
Regardless of your weight, if your navel folds in and disappears when you sit down, this is a good reason to select a different piercing. Wearing jewelry on this type of build causes mechanical stresses that guarantee healing complications. If you have a horizontal crease across the area even when you are standing, you are unsuited to navel piercing for the same reason. Some heavy people are still able to get navel piercings because body size isn’t as key a factor as your configuration in the area of the piercing. However, the diminished blood supply caused by excess abdominal fat does make healing more difficult. If you are diabetic and carry excess weight in this area, navel piercings are inadvisable because complications are to be expected and can be more serious.
If you are planning to lose weight, the overall shape of your navel area is still the primary consideration when deciding whether to postpone piercing. A protruding beer belly is generally suited to navel piercing because usually the area gets smaller with weight loss, but maintains the same basic contour.