There are at least three different variants of the lost foam casting process that I can think of, the common thread connecting them is that the molten metal comes into direct contact with a polystyrene foam pattern. The foam pattern is vapourized instantly on contact resulting in a void that's immediately taken up by the molten metal. The particular lost foam process used by GM with the ceramic slurry is the one that results in the best dimensional integrity and best surface finish so that the raw casting requires less machining to finish than could be achieved by any other non-permanent mold casting process, save lost wax.
The only way I could see lost foam casting being viable for short run or amateur casting of a water cooled cylinder head is if one had ready access to a CNC mill so that you could accurately machine a foam pattern, in several sections which could then be glued together to form the finished pattern.
The other use of foam you refer to, I would consider to be conventional sand casting just using an unconventionally produced reusable pattern.
Modified by ABA Scirocco at 2:05 PM 11-28-2009