I am not a plumber but had a similar solid fuel system in a previous cottage.
The plumber who installed it said that one rad would effectively always be on and hot regardless of the stat and pump. As I understand it this was to effectively dump heat from the hot water tank when the central heating was off. I think the flow for this was nothing more than convection..
In addition there was an over flow into an expansion tank in the loft similar to what is being described here.
This would have been installed about 30years ago, so I have no idea what the codes of practice were then, but after reading this thread I am glad nothing serious ever happened...
It is a heat leak radiator (to a certain minimum size for the solid fuel boiler MIs) to take some of the excess heat directly from the central heating pipes from boiler - the primaries, which are gravity fed.
It doesn't take the heat from the hot water tank, but from the source of the heat, the fire.
To be honest, if a solid fuel fire is really blazing, hot cylinder roasting and a pump fails, - one heat leak radiator isn't really going to take enough heat away.