I don't think they'd need artificial heat, just lag the coup floor well with wadges of newspaper and deep bedding. I've just done mine and its really snuggly in there, they were out, but have all gone in now to rearrang the bedding to their liking, as I never seem to do it right for them
It's quite snuggly in their, deep clean bedding, and out of the icy wind. And we're in a really cold area, high up on the Lancs hills. Quite a depth of snow this morning.
Also, if underfloor heating the nest box, might that encourage them to be broody? Sure I read somewhere chucks bums need to be kept cool? In fact I was told by someone with an Egloo that the nest box has a hole in the base for ventilation for just that reason. Not sure I believed them!
Only thing I'm having problems with is keeping the water unfrozen. Don't have power that far down the garden.