A common climate change denier argument is that back-radiation proposed by climate-change theory violates the second law of thermodynamics because it implies that a cold body warms a hot body.
My understanding of this argument is that it's incorrect because the cold body and warm body are both emitting thermal radiation, the thermal radiation from the cold body to the hot body slows the rate at which the hot body cools down and the thermal radiation from the hot body to the cold body warms the cold body up. When considered together, the overall heat flow is from hot to cold, no violation of the second law involved.
This explanation has made me wonder about the following scenario. Consider two black bodies of different temperature near each other in free space, connected by a unidirectional thermal radiation filter, such that thermal radiation from the cold body towards the hot body passes through but thermal radiation from the hot body to the cold body is blocked or redirected in some other direction. I'll assume that the filter is 100% efficient for this hypothetical example.
Would this system allow heat to flow from cold to hot? Would that flow be considered a violation of the second law of thermodynamics?