I would like to do some thing similar, I do see your point, the TRV control the boiler so all that is required is to turn boiler on/off. However in the main today we need to allow boiler to cool down, so we send an off command, but don't actually turn it off, in some cases this is extra low voltage, in others simply low voltage, and with the latter it may be possible to connect the device, it depends on the boiler.
However point one is you state you are already using a thermostat, this seems to be the flaw in the system, I don't know your system, so I will talk about my mothers system and you can then say how it compares.
The boiler has a time switch called a programmer, and from that there is a thermostat, also there is a programmable thermostat in parallel with the programmer, this is from a failed attempt at control but has anti-frost so has been left in place just in case the house should ever get below 12°C but in the main now does nothing.
So the room thermostat is in the hall, it has one function, to turn off the boiler when the day is warm enough not to need it, it work in conjunction with the thermostatic radiator valve, which will start to operate before the room thermostat turns off, so the room in this case the hall, will heat up quickly, until the TRV starts to work, once that happens the heating slows down to allow other rooms to catch up, before on a warm day getting the room warm enough to turn off the boiler.
The hall TRV and wall thermostat were very carefully set to complement each other. As a result once set the TRV and thermostat are not touched, it took a long time to set up, so anyone touching it needs ear plugs in to stand the comments from me. I actually set the stop in the thermostat to 19°C so if fiddled with easy to return to setting, also a felt pen mark on the TRV. But point is the thermostat does not control the house temperature, the TRV's in each room does that.
So to turn central heating on/off I use the programmer, the thermostat is always in circuit if the programmer is on. So to override the programmer it simply needs a switch. Some rooms have a set temperature 24/7, I keep bedrooms on the cool side, I have never liked a hot bedroom, the towel rails in bathroom and kitchen do not really heat the rooms that much, and again left on 24/7, however mothers down stairs bedroom and the living room do have programs running which is set in case of living room, 6 am 22°C, 9 am 20°C, 5:30 pm 22°C and 10 pm 12°C it never drops to 12°C but that is the setting, in the morning it never hits 22°C it is set that high so valve is held wide open so room heats up again quickly. Something similar for mothers bedroom.
So if we want to extend the time room is heated for we would need a wifi connection to the eTRV not the thermostat, the programmer does turn off whole heating at 5 am and back on at 6 am, this is to ensure the boiler is running when the eTRV's try to raise the temperature.
If it were not for my mother living down stairs where is gets cooler than upstairs as heat raises, then I could use the programmer to turn off heating at an earlier point. Maybe turn off whole heating at 10 pm.
I did earlier try using a programmable thermostat which was portable room to room, idea was to vary heat through the day, however it was a failure, it is easy in hind sight, but I thought a thermostat in the room would control the main room temperature, however a thermostat switches on/off, it may be possible to design a thermostat which slowly changes a mark/space ratio but the ones I have used, simply switch on or off. The problem is the further the thermostat is from radiator the greater the hysteresis, but put it close to radiator and the radiator would heat the thermostat prematurely, although one can set to a higher temperature to compensate this means come a warm day and the room can get stinking hot.
In contrast the TRV is gradual in operation, so once set the room will stay at a constant temperature, the TRV also controls how hot the return water is to boiler, so with a modern boiler the flame height is varied to suit the demand from the TRV. So allowing the TRV to do it's job works well.
However a standard TRV only measures air temperature so is to some extent effected by the hot water in the radiator, however simply changing the head to an electronic one, then they measure both air and water temperature and correct the reading to allow for the heat from the radiator, also they can be programmed as to when to heat the room.
The eTRV comes in three flavours:-
1) Stand alone, with no wifi.
2) With wifi which although it can be set to do automatic functions, it requires "If this then that" (ITTT) to link with other devices in order to control the boiler.
3) Designed to use a central hub so boiler is in turn controlled from that hub.
Although mine are the second type, they are never altered so could have been the cheaper first type. It is rather annoying the web page clearly shows current and target temperature and if the current is below target with any eTRV we want boiler to fire up, to get a switch to use this information seems near impossible, relays are made which can be also controlled using ITTT but as far as I can tell you can't write your own programs, you can only select those written by the manufacturers.
There are to be fair 100's of pre-written programs, however even though you only want a on/off device, it seems you need something like Hive or Nest to be able to set them to follow the eTRV, however once you do have a hive or nest then the simple matching of the wall thermostat and TRV is completely messed up, as to being able to set heating to either be manually set with phone or automaticity sense how far away from home the phone is and so alter the set temperature can be done without having a hive or nest.
So in real terms to control heating away from home hive and nest are useless, except of course when using hot air heating systems for which they were designed, or with open plan houses.
So in real terms we have a huge jump in price from type 1 and type 3, with the third type there is no argument it does really control the house, there are a number of makes, look at EvoHome as an example, this does work well, but the price reflects how good it is.
So I can see how you could use a simple relay to override a simple programmer, however not so sure if you could use it to override a programmable thermostat? I would say you need to return to a separate thermostat and programmer to do what you want.
I used MiHome the eTRV works reasonable well, but I found the sockets and light switches are limited to three times on/off.
