How to reheat sauce made from cream reduction
I attempted the approach suggested in @moscafj’s answer, and heated the sauce in a bath at 60C. The sauce broke horribly, and I couldn’t re-emulsify it with an immersion blender. Adding in more cold fresh cream did not create an emulsion (as it did the day before).
At this point I was at least somewhat stressed, but I was able to cool down by fanning by my face with the gentle breeze from the blades of my immersion blender. Then I added some Dijon mustard, and an emulsion formed nicely.
In the future I want to try reheating the reduced cream and mounting the sauce with the cold cream right before serving.
A safe option seems to serve the sauce at room temp. I held back some of the sauce when attempting to reheat it, and found it was stable, smooth/pliable and tasty at 25C.