Jpen, I think what coaledsweat is suggesting is the same as below, just re-worded. If not here's my take on your plumbing schematic.
I would not use the oil boiler to heat a storage tank, use the external coal heated coil to heat a supply tank[also called a tempering tank]. The way you have the plumbing now, you will have a large quantity of oil heated water cooling-to-tepid in a storage tank, and no way to raise the temp of the stored water quickly. The effectivness of an external coil is still a big question, and you are gambling that it will keep the storage tank warm. But you will still be using oil all the time to heat any domestic hot water. If you shut down the coal stove the water in the storage tank will cool within a day, and you won't have hot water.
Use the external coil to heat a supply tank. This supply tank can be a discarded hot water tank, either 40 or 50 gallon is common and will work. This supply tank has the cold water supply to your domestic hot water tee'd into the loop. The outlet from the tank/loop goes to the inlet of your steam boiler's water coil.
The way this will work is that during a long night or day that the coal stove is producing heat, the coil will slowly heat the water in the supply tank, and store this hot water. When you use domestic hot water, this preheated water will supply the coil in the boiler, then on to the point-of-use in your house. If the water in the supply tank is hotter than the boiler's water, then some heat will be transfered to the boiler, can't be helped. This heat is still stored in the boiler water. If the supply water is cooler than the boiler water, then the boiler will eventually turn on the oil burner to keep the water at a desired temperature.
Your external mounted coil is not going to be very effective [my opinion], and the sand-box you described may act as an insulator instead of a conductor. I'm thinking that this location is going to make the coil almost ineffective at recovering heat. These coils don't have much surface area, and need to be in the high temp of the firebox to be effective. I'm thinking that at best you will be warming the supply tank, I doubt very much if you can get 40 gallons of water to 200* in even 12-18 hours of heating with no added make-up cold water. [no hot water use].
If you can find a discarded electric hot water tank with a bad electric element, and make up your own coil, you won't have much invested if the external coil is ineffective. Experimenting is a good thing as long as there isn't too much money invested.
Any amount of pre-heating the supply water to your steam boiler's coil will save you oil, even if the water is only 100*, it is better than the usual supply water temp of 50-55*. So I think it is a worthwhile experiment.
I think an insulated aluminum or SS box around your external coil with a highly polished inside surface would be more effective at transfering heat to the coil than surrounding it in sand. Most of the heat coming off of the back of your coal stove is radiant, and needs to be reflected back to the surface of the coil. The surface of the coil needs to be a dull dark color, to absorb as much heat and reflect as little as possibe.