Whether you consider it to be a new siege mechanic, or an aesthetic property, I think that this would be a great addition.
Sign post with 25-50 dirt = 1 tile of ridge leveled out flat, with filling in a mine hole costing like 300~ dirt or not being able to be filled in at all preferably since that can lead to even worse griefing of new players or lower capacity players who have a hard time putting up mine holes. In the same sense, if you allowed filling in a cave, why not be able to fill in mine holes so that illegal vaults are harder to make? You could also have a feature so that filling in a cave or mine hole gives a 12-24 hour grace period where the soil "settles" and can be undone, just like setting a ram. So if a newb gets raided, chances are unlikely that some high tech player is gonna sit around for the entire 12-24 hours just to be that big of a dick.