I am not too familiar with c. myrtle, but I quickly checked some sites and it said hardy to zone 6 at best. Are you planning to overwinter outdoors? If so, perhaps you can try a method I use to overwinter japanese maples. It's sort of like overwintering tender roses.

Basically, I take a large pot (maybe 5 gallon cheap black plastic, the type a tree comes in from the nursery) and cut out the bottom. Place this around the shrub. Fill with peatmoss and then with leaves until shrub is completely buried. When it snows pile lots of snow over top. Alternately, you can use a cardboard box with the top and bottom flaps cut out. Anything really, that allows the peat moss and leaves to pile up over the plant.

If they are in pots, another method might be a refridgerator. I had a small ginkgo tree that, once it lost it's leaves, I put in the fridge to simulate winter. It seemed to work alright, except for taking up fridge space. I watered until lightly moist and then wrapped the pot with plastic wrap and placed on it's side to save space.