Saturated? I think you can actually mix CO2 and air in ANY concentration without saturation - at least at standard temperature and pressure.

And while CO2 separation from air is handled rather readily by any LOX plant, sequestration might be a bit more difficult. Venting the O2 wouldn’t be a problem, but storing the dry ice would be.

With cheap fusion energy, you’d probably do better just taking methane and making methane hydrates of it and pumping it down into the Marianas Trench or someplace cold and under pressure..