Banking a fire with ashes is an old method of slowing and holding a fire over night. My father told me that was one of his jobs setting up the coal furnace for the night. Come morning just open the dampers, shake the ashes down and the fire recovers nicely so that you can start adding fresh coal.
It's also very common to bank wood fires, too. I'd often do that at night when I used my fireplace and we do that with our camping fire pit. If buried in ash right, there's plenty enough fire left to easily get it going quickly the next morning.
I don't bank the coal stove fire with ashes or fines for over night, but I do use a shovel of fines placed in the middle of each round cover opening to slow a too-hot fire. Acts like putting a large piece of coal in the center and it works great at slowing, while still adding fuel. And it's a good way to get your money's worth of heat out of all those fines you paid for.
To slow and extend overnight fires, when I'm getting the stove "ready for bed", I use a higher concentration of smaller pieces of coal. It works similar to blanketing with fines in that it gives a high concentration of smaller pieces not only increase the fuel density in the firebox, the tighter air spaces also add air flow resistance up through the firebed. Both contribute to slowing and lengthening the burn time. Using a blanket of fines just takes that to another level and slows it even more.
But, as Lee said, you have to leave some open space around the outer edges. Cover the firebed completely with fines and you risk smothering and putting the fire out.