if you crunch up the foil until its solid ( i use a rolling pin to help smack it into shape) you wont need wire but you will probably need more foil than you thought you would especially if its going to be large.
usually with ceramic you can bake the clay straight onto the piece though something like this may be fragile (i dont know much about dolls heads) but polymer clay shouldnt shrink when baked. you can sand away and reshape the baked clay if the fit inst exact anyway. i tend to use a reasonable thickness when baking over foil if only because if you go very thin you tend to get bits of the sharper foil poking through the clay. i tend to use a Cyanoacrylate glue such as super glue with polymer clay to attach cords etc