Yeah, I've discovered a similar problem trying to find the heads and faces. They're apparently labeled differently than the other skeletal meshes for an in-engine organization reason as the heads are usually identical but with different textures and accessories. I have found the base head mesh through some tampering, but I still can't find the base alien head, the head for the Ethereal's, and the head of Origin the main antagonist. I DO know which files contain them, but as for extracting them I can't seem to find a way. They're clearly mesh objects as there are animation files referencing them, as well as 'FaceFX' files corresponding to them. I've gone into some depth figuring this out but it still escapes me on how to extract them properly, due to their 'custom class'. When extracted they come out as 'MorphTarget' files. Here's a pack of some upk's and their extracted contents to have a look at.
https://www.dropbox.com/s/io8g5d1i820taum/BurEXT.zip?dl=0