Hi Dan and a Happy New Year to You and Yours.
I can't say whether Feb Wintamix, on it's own,,,, or "car antifreeze" would work in external render coats in freezing weather... With brickies/builders, they are also able to protect and cover their work with hessian/polythene etc as they build, if it gets too cold outside, but it's not easy so covering up a rendered wall/walls. I personally wouldn't take a chance rendering at this time of year as I said,,, but others on here might offer different advice. Plastering/rendering, water based painting, etc,,, even "inside" an empty building, can all be ruined by extreme cold weather too, without using some form of heating/protection...
Roughcaster.