I use IFTTT using Google calendar and Evohome. (When son is on Day shift at the weekend) I make a calendar entry with the title 'Day Shift' and created an Applet for each Evohome zone. Then raises the temp for an hour in the zones I want.
Can you use something to make a calendar entry if the temp falls below -2c then make an applet to pick up the calendar entry to do the evohome bit i.e. do IFTTT in two stages or use stringify first and then IFTTT for the second bit.
I haven't solved it for you but it might help you work out a way of doing it.