Does anyone see any reason why I should not connect the froststat across the 'live' and 'CH on' terminals on the Hive backplate?
That would be the normal way to do it, with the provision that your existing wiring to the Hive contacts is 230V and not the 24v used by some boilers.
The problem would come when the temperature drops, the frost stat turns 'on' and if it is a prolonged spell of cold weather the Hive will be completely overridden and the frost stat may keep your heating on 24/7. Then, if, as it should be, there isn't a radiator with a TRV on in the room where the Hive thermostat is located it's gonna get pretty warm in there; especially if all of the other TRV's are closed.
To prevent this happening, the frost stat is normally wired in series with a pipe thermostat set to about 20 degrees, and fitted to one of the return pipes at risk. This means that the circulating water in the pipes is kept warm enough to stop the pipes freezing, and radiators at 20 degrees won't overheat your house.
However, as the tank you refer to is for cold water, heating the house below with a frost stat and hoping sufficient heat permeates through the insulation into the loft to warm it, is a bit unreliable, and it probably wouldn't add anything more that the normal heating of your house does on daily basis anyway. In this case ianmcd's suggestion would probably be a better idea. Make sure you get a suitable electric heater though like a tube heater or similar so that it doesn't pose a fire risk.
I have to say that I have a 4 gallon F&E tank right up in the apex of my roof. It's been there 20 years and has the standard plastic bag and rockwool type jacket on, not had a problem to date. Been up a few times when we've had -10 degrees for a few days, to check it, but have never seen any sign of ice. The cold water feed pipe is extremely well insulated though, as that is more likely to freeze having less water in it.