I don't believe that the nations.txt breakdown is used any longer. It basically goes:
NATION
|_ EACH ETHNICITY FOR THE NATION|_ EACH ETHNICITY'S RACIAL BREAKDOWN
So you essentially have to control the FaceGen stuff at that third level. The fields in nations.txt are probably legacy fields from past versions of OOTP. It's better at the ethnicities level since you can control the nameset that goes with the picture. If you did it at the nation level, you couldn't prevent Junichi Tanaka from having an African FaceGen picture.