Also: where was all the carbon hundreds of millions of years ago before it was turned into fossil fuels? I guess "steady state" is a relative term when you get to long time periods.

Your ever-growing compost pile might be analogous to peatlands, but on the other hand it will probably just be returned back to the atmosphere in the next 100 years. The future occupants of your property are unlikely to continue your experiment :)

My guess is that anything we gain in that regard is more than offset by all the land that's covered in asphalt and concrete and also modern farming techniques and general deforestation. Very little carbon sequestration going on in the modern world..