This device is very difficult to explain but the jist of it is that a mass of a clay like material (moist/soft clay, not baked/cured) needs to be carefully removed from a cavity with very thin, flexible and "fragile" sheath. The size and shape from the mould can vary a good bit but anywhere from 2"-8" diameter and 2'-14" long - with dimensions a combination of any number in those ranges and it can be of varying diameters along the length. It is difficult to explain, just imagine a balloon filed with clay that can be squeezed in random areas to get unique shapes and a balloon is very similar to the sheath material.
So the clay-like material needs to be carefully removed. The sheath may be empty for 2-8" before the clay starts and the sheath is water-tight. This tool will almost definetly need to be a flexible shaft/tube to reach all material and also to get through narrow sheath openings.
The process need to focus mainly on maintaining the integrity of the surrounding sheath and material removal can be slow and meticulous if need be.
Water and oil (mineral and some others) can be used in the process, to help keep the clay from sticking to removal tool. The material will be able to be removed through a tube (probably from 1/2 - 1" OD) and water/oil injection can be used to create flow of liquid to allow the material to be flushed out the tube.
I was thinking of a silicone coated plastic or metal auger (maybe 1/2" diameter) with a cylindrical tube encasing the auger. As the auger turns an outer casing is extended to surround the material the auger has dug into, the auger could possibly pull it in as it turns. As the auger turns the material is fed back into the tube where it will need to be "flushed" back out of the tube and extracted. I though this could be done by some water jets coming out the side of the center post of the auger (maybe 1.5-2 turns from where the auger starts, so there is material in front of the water jet keeping it from pushing the material & water forward in the tube and (the "high pressure" would break up the material and the flow would push it back through the tube to extract it. I can picture this clearly and have seen some drill rigs like this but nothing small for sensitive areas.
Another method would be to somehow extract chunks or bites of the material with a rotating 1/2 cylinder with a domed top to gnaw away at the mass, then retract the cylinder into the tube where it would be scraped off as it rotates and flushed down the extraction tube via suction and water flow into the sheath cavity.
I'm sorry I can't be more specific about the process but it is not anything illegal and it is of a very sensitive nature as far as the procedure.
If anyone knows of any method that would allow for chewing away or breaking up something like this while not damaging the lining/sheath, I'm open to any suggestions. It will be possible to have a small camera on the unit to see what is being done while the process is being done if that makes a difference.
Things that can not be used are solvents, hot water/oil (only 80-100F water/oil can be used). high pressure outside of the collection tube, sharp materials that could cut the lining (why silicone coating was suggested
You might consider a two stage unit with the steel cutting auger you proposed, sized blow the anticipated minimum diameter (8" or what ever) of the mold, and connected behind that on its shaft a two or four bladed section of scrapers with their outer sections made of a flexible polymer and are curved backward can flex to match the mold sheath varying diameters. With the core of the soft material removed to weaken the strength of the remaining outer cylinder of the material it should be easier for flexible scrapers to collapse that outer cylinder of the material to the center. Water injected behind the front cutter can then flush all of the cutting from both stages out of the back of the unit.