I know from the airfreight business that in general, if not stated otherwise, the temperature in the a/c belly is 10° C.

But I would never count on that... There will be times when suitcases are in the airport baggage system, they are on the maneuvering area or the temperature in the belly of the aircraft is different because they have dangerous goods on board which need a curtain temperature or for animals or whatever.

So, I would do the same like the former posters adviced, take it as a carry-on and ask the FA's to deal with that stuff...