Can large pieces of slipware be dried without a kiln?
Making a piece out of ceramics, you should use plaster for the mold as it draws out the moisture from the clay slip, and you will not  need to  wait a  long  time to get the thickness of your  clay.  If you use a rubber mold, the moisture has to leave the clay through the  air, which is very slow.
Raku is a way to fire the clay, so unless you want a hardened piece out of the clay, I would not fire it raku style or otherwise.
If you are wanting to make a bronze piece, why use the ceramic as part? You can make the finished piece in several parts of bronze or clay.  The pieces  can  be  combined  afterward; molded clay glued with slip, or welding or soldering the bronze.
Another way would be a breakable mold for the entire piece, where you break the complete work out of the mold in one piece.