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..
If the amount of carbon in the soil is secularly increasing at 0.01% per century, well, that's maybe interesting geologically, but for the purposes of handling climate change in the next century or two, we can treat that as equivalent to "it's steady state."
But I guess that timeframe isn't very relevant to any climate change we'll be experiencing in the near future.
While the percentage may remain constant in the steady state, why doesn't the soil mass increase? For example, I presume the total mass of carbon in the soil is greater now than it was prior to the evolution of green plants. At what point did it transition from accumulation to steady-state?
I'm not saying it didn't, just that I don't understand the logic that it must have. For example, I'd guess that there is more absolute organic matter in the soil in the northern US now than there was just after the retreat of the glaciers. Is this false? In the absence of erosion (and human agriculture) I'd presumed this process was still continuing.