It is indeed possible and on an Economy 7 system I had down in Southampton back in the 90s, it used a Sangamo mechanical time clock to switch the immersion heater and storage heater loads.
The problem with some older systems, especially using Radio TeleSwitch (RTS) was that the heating and hot water was often on a different tariff to the regular Economy 7 system.
Plans such as Heatsave or Flexheat (popular in Scotland or in Southern England back in the day) had four tariff rates. Standard Economy 7 with peak and off peak rates for your household electricity needs, and then two separate rates for off peak heating/water and a daytime 'boost'. These used complex metering arrangements with four rates and two sets of standing charges. SSE was formed by a merger of Scottish Electricity and Southern Electric as they were the only two companies that rolled out these systems.
Anyway, a correctly configured 5 terminal meter with the four separate rates is indeed possible but is something that only SSE and OVO seem to be able to get their heads around.
A standard four terminal meter will physically do the job with a timeswitch for the heating loads, but there would be no provision for setting different rates specifically for heating use.
The South of England used to have a very ample supply of off-peak electricity thanks to input from Winfrith, Berkeley, Oldbury, Hinkley Point A and Dungeness A Magnox nuclear reactors so it made sense to use the surplus night-time generation for heating and hot water. Remember the old Heat Electric adverts from the 70s?
Similarly in Scotland. A very much smaller population, largely off the gas grid and ample overnight energy from Hunterston A and Chapelcross reactors.
With all the older Magnox plants now in decommissioning, all that 'spare' off peak power is a thing of the past so schemes like Flexheat and Heatsave haven't made sense for over twenty years. Most of the legacy systems should have been ripped out and replaced years ago, but there are about 1.7 million existing installations, 1.2 million of which use Radio TeleSwitch which is being shut down in April next year.
Don't shoot me, I'm only the piano player. I DON'T work for or on behalf of EON.Next, but am willing to try and help if I can. Not on mains gas, mobile network or mains drainage. House heated almost entirely by baby dragons.