OK, I apologize for obviously giving this way too much thought. Remember tan lines are caused by clothes. The clothes are not painted to a person. For instance a bikini moves, and doesn't adhere to the body in low spots. Personally I'd use a MAT. Tan lines need a bit of blurring, and reduction in strength in key areas. For more realism, two or more sets of tan lines should be combined but slightly offset; this gives the impression of several tanning sessions.