just don't let the water get in contact with the firebox itself. Might need a little shed roof to keep weather out of the stove, and store some dry
firewood nearby.
The water itself will have way more thermal mass than your
cob (about 3 to 4 times as much by volume) so you shouldn't need much masonry if you can insulate and weatherize it.
seems like a sound idea in principle.
You could even stage a pot of
hot water on top of the barrel, with a tap on the side to drain it into the tank and heat things up that much faster. You shouldn't need more than a 30 to 45 minute fire if you are dumping
hot water in directly to mix with the cold in the tank.
If you want to insulate the barrel, don't - but you can heat shield it with air gaps, and force more of that heat up around the pot.
It does seem like a chore to trek out there and fire up the heater, tho, which is probably why people go to electric. You can just leave them on, sucking little dregs of power off the grid, and you keep your toes warm indoors.
If this is right near your house / barn and you go there daily anyway, I'd be tempted to rig things so you get a little more heat into the building from it and just use the hot water from atop the barrel to heat the stock tank. Make a warm spot the stock hang out in, and they may also serve as thermal mass / buffers to reduce the heat loss and keep their water thawed.
-Erica