If you're going to put a coil inside the heated containment, the top of the first bell is the place I would choose. Near the riser is too dangerous - it would be likely to sputter steam frequently at the very least, assuming you had the outlet to the storage tank in open air above the tank, and only a few feet from the bell. Any obstruction that happened somehow would cause an almost instant steam explosion there. The bell would have similar but much milder risks. I saw a
video of a
hot water setup similar to this with
hot water and a few steam spurts coming out. Okay if you plan for it properly.
The safest method, as described in a number of threads around here, would be an open tank set into the bell with a coil going through it; this prevents the water in the coil from ever getting above boiling point, and if it is pressurized at all, it could never get to the boiling point (pressurized water has a higher boiling point than atmospheric water).
This setup will require the RMH to be lower than the reservoir if only by a bit, of course. Are you also planning on a
solar water collector for heating the reservoir when the sun is out? The combination of sources will give you a lot of edge days where you don't need to light a fire at all, and it can take care of most if not all of your domestic hot water as well. For maximal freeze-safety, I would advise a drain-back system. It requires a pump to circulate the water to the collector, but in a power failure the water will just go back to the reservoir leaving nothing exposed to the cold, and not requiring antifreeze. The collector in this system needs to be above the reservoir by at least a few inches - more is not helpful.
