Buffer Tank AFTER load?

I've almost got our used and abused system to a good place. The last piece of the puzzle is a buffer tank for some small radiator loads. Due to space constraints, the only place to easily install a tank is on the return to the boiler, after the radiator manifold. I'm thinking about taking the manifold outlet and connecting it to the top connection of a Westinghouse stainless water heater. The bottom connection of the Westinghouse will head back the the heat source. So, the heat source will heat to 150, lose 20 at the rads, then fill the tank from top to bottom with 130 water. Once the bottom of the tank reaches 130, the boiler outlet will be pushed up into the control offset, then finally turn off around 160. The circulator will remain until the zones are satisfied. I'll size the tank for around 20-30min burns, hopefully more.

I don't see a lot (any) setups like this on the internets. Is there a reason why? What am I missing?