Hengeyokai, Shapeshifters of the East, page 43 contains this paragraph, right before the Tapestry:
A list of all the physical locations of interest in the Mirror Lands would take several exhaustive volumes, but one region deserves special mention. Swirling mists fill the border between China and Russia, and a great barrier stands between the two countries. A powerful force, spoken of only in whispers, separates the Russian Penumbra from the Mirror Lands, and travel through the spirit world between China and Russia is not possible. Many shen stand sentinel on this border, diligently guarding against whatever threats may break through from the other side.
Someone mentioned that this would be purple prose for the Shadow Curtain in Russia, but the problem is, that Rage Across the World, which describes the death of Baba Yaga and her Shadow Curtain, doesn't even mention this barrier to China. The Shadow Curtain was also described as increasing the gauntlet to nigh unusability in Russia, not as a swirling barrier that bars passage over one border. To me, this makes it at best a possible match, but not a confirmed one with just that text.
Is there any other mention of this barrier or further information that would verify that it might or might not be the Shadow Curtain?