If you wanted to clarify dry ice, you would have to put it under sufficient pressure to fuse all the crystals, much as one would need to put snow under controlled pressure to convert it into clear ice. Of course, it is tricky to get all the air inclusions out, but it is not impossible in principle. I have never seen it done with CO2, but in theory it could be made to work.

An alternative approach would be to cool pure, supercritical CO2 progressively till it crystallises around -56C. But it might be a bit tricky to avoid an optically noisy crystal structure.

I wonder... maybe crystallising CO2 under pressure from a solution in ether?

Sooner you than me!

Jon