I've never tried using $hipslot, but I had a similar problem with general image mounting a couple days ago, regardless if i used a direct number or $headslot/$backslot.
If I remember right, using mountimage(ImageName, $slot); didn't work, but when I used mountimage(ImageName.getID(), $slot); it worked fine.
This may or may not be the problem, but you could give it a shot anyway.