metal or ceramic foam

That is the crux of the problem. If production has to be in a gravity well or a centrifuge well, energy must be used to get the material in and out of the well. Perhaps, but I don't think the energy requirements of the latter can even begin to compare to those for the former. There are also size restrictions on material transport whether that is the throat size of a mass driver or the elevator size for a centrifuge. The diameter of the mass driver is sized to make its emplacement a realistic, near-term possibility. And it's important to remember that there's a continuous flow of materials (well, maybe for 2 weeks out of every four). It's a pretty narrow pipeline, granted, but you might be surprised how many tons you could get through it over the course of five years or so. I'm sure there will be many bottlenecks, but I just can't see the elevator size of the centrifuge as being a major one. In later studies, emphasis was placed on systems that could replicate themselves, so we would be talking about a greater number of elevators in a greater number of centrifuges as time went on. In the High Frontier as well as other lunar / asteroid resource utilization scenarios, there is always a sentence that states that the bulk material is used to create fabulous things in zero gravity. Now NASA has never been interested in actual manufacturing in space. The few experiments that have been performed have been done by ESA. There is much more information on mass drivers than on the system that would actually use the output of a mass driver. I consider not being able to show several realistic designs for a zero gravity material processing system a major problem for any lunar / asteroid resource utilization scheme.
That's a good point.