Well, there are on the new red axs chain sets.
But now that you mention it, no one really does aftermarket 12 speed rings yet.
But I suppose it's just a case of manufacturers catching up with what is a brand new product.
My though is that the new red axs crankset is really expensive and ugly.
There's going to be folk running hollowgram cranks and spidering who want to upgrade to sram 12 speed at some point.
I'd imagine a sub compact spidering must be in the pipeline.