Well first of all I edited 2 models (made one headless and one... bodyless) and when I geomerged them (I think I attached the second model to the correct bone of the first) the unit in wc3 viewer it still is headless but it has a 2D square attached instead of a head.
P.S. I didn't use a texture in the geo merger and the headless part still appeared normal.
P.S. I didn't use a texture in the geo merger and the headless part still appeared normal.