There's not much I can do about this one, other than what I'm already doing.
Yesterday it was 97 F (36 C) for a daytime high, my rhubarb seems to have survived that. Today, the forecast is for even more heat, high of 100 F (38 C), with sustained temperatures over the next 10 days hitting 36 C to 40 C (104 F) every day.
The rhubarb is still looking good for now, and I will keep the pots in full shade, checking water and signs of stress at least twice a day. If I can get them through this, I will feel a lot more confident regarding the conditions I can get them through in the future.
It is interesting that our advanced forecast for July shows cooler temperatures, with forecast highs for the month only 90 F to 96 F, also coming with more chances for rain.