I would think that broadleaf evergreens in containers on a windy balcony would be quite the challenge.
I think Pieris like an acid soil. Maybe wrap the plants in burlap to protect them from sun & wind?
It will be an interesting experiment.

The Pieris I have seen in Lansing, kept some leaves all season. They were in protected locations but not getting the sun they needed. Looked kind of scruffy. It is kind of an iffy plant here in zone 5, may not do real well. I will agree hearing they need acidic soil.

I have some things I put protection around, but I have used clear plastic. I have used the plastic wrap that you see on pallets shipped into warehouses. It is kind of like wide Saran Wrap, sticks to itself a bit. Usually easy to get at a Feed Store or any big box store, Meijers type place if you ask at the back.

Another option is old used shower curtain liners. Usually wider, longer, not sticky.

I like the plastic for stopping air flow, still allowing light on the plant in winter conditions.

I use the plastic to surround my EXPENSIVE, SENSITIVE Japanese Maple, Oakleaf Hydragea. Both are easily wind burned, which kills limbs and flower buds. Surrounding the plants is pretty open, not tight or wrapped up. More of wind break factor. I don't cover the tops, snow load would collapse the cover. Open top also allows any heat from sun to escape quickly, don't want plant to break dormancy.

I will admit to being very happy with the plastic protection. Cheap, Easy. Doesn't take long to put up or take down in season. I only have the two special plants. Potted plants could be more tricky, not got the dirt protection of plants in a bed. Easier to freeze and thaw quickly.

You might consider additional clear plastic across the balcony railing, as a wind stopper, double pot the plants for bigger insulation layer. Best of luck over the winter.