Hey, wasn't able to try this until now. But anyway, it's driving me nuts and flowers.
After weighting the one piece model, I duplicate it (and delete the skin modifier that was duplicated along with it), and I detach and rename body parts. When I paste the skin modifier of the one piece model, each body parts snaps into the approximate position of where it was duplicated from - except the head. The head goes batpoop insane and places itself wherever it likes around the workspace. I can't move it back into place, because when I do that and finally get it near where it should be... it snaps back to the same or some other random place far away once I deselect the head, or the skin modifier. This MIGHT have to do with a possible change of pivot point long before pasting the skin modifier... but I'm not sure if I did.
Anyway, so the head refuses to be placed.
So the difference between this, and what I did before, is that every body part each has all the bones in the body in the bone list? I guess that's what happens if you copy the skin modifier of the whole body and paste it on each limb.
I can try going to an earlier save of the project and see if it behaves the same... but if there's an obvious reason why this is happening, I'd be glad to hear, if you know