But you could cast a negative from your mold, then coat that in mold release and make a mold from that in a harder material for your polymer clay - the downside is that many of the molds will become one time use if you use a material that is TOO hard.

I wonder about freezing your molds at very low temperatures - that could make them hard enough for teh clay, but they may become too brittle.

I think you if you wanted to pursue this further you should look into slip casting.