No there is not. You misread. It was in Saints Row 2, i didn't find this flag anywhere in the Third but I tried it anyway and it didn't work.
It may not even be the same skeleton.
In the mission (SPOILER!) where you look like another character, it's just a special headgear and armor and they even seem to apply another skeleton to it.
From what I've seen, the only clue (and it's a big one) about the possibility of playing as an NPC is the Be all you want to be code which should show a list of characters to impersonate but doesn't seem to work. But I think its presence is enough to continue the investigation.
It may not even be the same skeleton.
In the mission (SPOILER!) where you look like another character, it's just a special headgear and armor and they even seem to apply another skeleton to it.
From what I've seen, the only clue (and it's a big one) about the possibility of playing as an NPC is the Be all you want to be code which should show a list of characters to impersonate but doesn't seem to work. But I think its presence is enough to continue the investigation.