Allowing stolen Boeing aircraft to fly in Chinese airspace is a clear sanctions violation. I think there would be a significant negative response from the US government which will directly compromise Chinese global business interests. China is, like usual, playing both sides of the board but not to the extent that support for Russia has a significant cost to China. The proof of provenance requirements is IMO a quite elegant solution. It will in practice deny Russian aircraft entry into Chinese airspace, thus placating the US but without directly targeting Russia as they can tell the Russians it applies to everyone.