A modified re-breather loop is actually not a bad idea.
You would not need CO2 scrubber material or an oxygen supply. You only need to make the loop out of a suitable gas-permeable materiel, that will equalize CO2 and oxygen concentrations with the outside air, via molecular exchange through the gas-permeable barrier. It's basically a filter that needs zero air-flow, because it can exploit Dalton’s Law.
<The Pressure of a Mixture of Gases: Dalton’s Law<Unless they chemically react with each other, the individual gases in a mixture of gases do not affect each other’s pressure. Each individual gas in a mixture exerts the same pressure that it would exert if it was present alone in the container. The pressure exerted by each individual gas in a mixture is called its partial pressure .
Basically the air you breath out into the re-breather-loop only has around 16% oxygen left in it. The partial pressure for oxygen inside the re-breather-loop would be much lower than the outside air. Pressure equalization would draw in lots of oxygen-gas through the gas-permeable material. The same would happen for CO2 but in the other direction. Gas-permeability only needs holes large enough for molecules to fit through, viruses are huge compared to molecules.