How much heat can a demijohn take?

I'm planning to serve some homemade cider, hot and mulled at a market stall in the coming weeks. I'll be siphoning out of a 23L demijohn (like this: http://www.toowoombahomebrewers.com.au/Wine/Accessories/glass-demijohn-23-l to serve.

The plan is to heat the cider in a big pot to the right temperature (say, 70C), pour a little into the demi to let it adjust, then fill it up. I might need some sort of heat source to keep it warm, but I won't have power. I'm thinking about a very low gas flame with a lot of diffusion (fine metal screens) underneath the demi. Then again, 20L of hot liquid stays hot a long time, maybe I won't need to worry.

Am I mental? Am I going to explode the demi? We clean them with a hot-water pressure cleaner, and they're fine, but I wonder about having them full of hot liquid. Last thing I need is 23L of scalding cider exploding everywhere on the night.