I took some time to examine the model bones again. Just to remind everyone, this is the reason why the FIFA exporter is not capable of exporting custom meshes as head, hair or boot models.
I managed to find the bone head positions out of the transformation matrices, but that does not seem to work will all the bones. That is still a heavily WIP.
Despite that, it is nice to know how many bones control a FIFA 15 face (and they are more than 30).