Hello Bill,
You should be fine with temps in the 50's. I have large brugs in the basement right now. They have been there since I brought them in in October. They don't get any artificial light, and the only window they get any form of light from is from 2 block glass windows. The only thing that I think you must be carefull about now is watering. They wont be growing as much and there for they wont need so muchc water. I water my large brugs in the basement, maybe 1 time every 2 weeks. I cannot express the importance of checking the soil moisture levels though. I have 5 plumeria this way and I was sick over it. The reason I lost them was from being too wet. I had assumed that they needed to be watered 1x a week, really was an amature mistake!
Andrew

Every year I keep large and small rooted cuttings in my
attached garage. (much lower than 50 degrees and they do
fine)

Hi Bill-
I have all of my brugs in an unheated basement at 52*. My new cuttings are on a heating mat, and others are on the concrete (cold) floor in a bucket of moist Perlite. Both are rooting fine and putting on new growth and roots. I've just read that if your brugs are used to lower winter temps, you'll be able to put them outside when the outdoor temps equal your indoor temps. Must be that they're hardier and able to withstand those temps. How fortunate you are to have a GH! Best Wishes-
kasha77

I currently have rooted cuttings in a 40 degree greenhouse in a peat lite soil mix in 4 inch pots. They are watered only when very dry, and are doing just what I want them to do, which is to grow slowly. Once the heat is up, as you know, they will grow very fast!