This isn't really a single-box solution, but it may be about as close as you will get and it doesn't require a computer.
If you can find an old thermostat, the kind with mercury bulb switches, you could solder long wire leads to one of the mercury bulb switches and connect the other ends of those wires to the terminals on an X10 PowerFlash interface (model PSC01 or PF284). On the PowerFlash, set the Input switch to A (dry contact) and the Mode switch to 3 (single unit) and set the House and Unit code dials to match those of the wall switch. Set the thermostat dial to the desired threshold temperature. If you are using the mercury bulb switch for heating, an ON command will be sent when the temperature falls below the set point and an OFF command will be sent when the temperature rises above the set point. If you are using the mercury bulb switch for cooling, the commands are reversed.
Note that this kind of thermostat must be kept level; tilting it will offset the temperature set points and any jarring or vibration could affect its operation. This kind of thermostat was very common until recently and most thermostats that use a mechanical sliding dial to set the temperature are set up like this.