Lets look at your example, and simplify it to a flat panel, with a picture of David on it.....hope you don't mind:) You would first need to reverse your .svg image, and as a result your carving, so that the cast from the plaster mould would be a positive. If you were to then make an aluminium cast from the plaster mould, the David cast would be a positive in metal......hope that makes sense.
Further to my previous post......a plaster like 'Crystacal' works really well on a CNC, but you've got to make sure, 1: it's fully cured, (I would cast blocks in advance, flatten them, and leave them wrapped in paper towels, in a dry place for at least a week), and 2: you take it steady.....don't try to remove too much material on each pass.....if you do you will have blow outs!!
I love blue machining wax........I made a lot of metal prop masters from it in the past, and my word it holds the detail really well!!!
All the very best