if you install a rainshower?

The only treatment your ceiling should need is a good quality paint job.

The ceiling should bot be getting wet if I understand you correctly.

The rainshower shower heads rain down.

The moisture in the bathroom can be a problem for ceilings hence the good paint job.

All bathroom ceilings that contain a bathtub or shower should be painted as follows...

first a good cleaning, 1 part bleach to10 parts water. Wipe the ceiling good with a sponge or rag.

Allow to dry.

Prime with Kilz2 if necessary.

If the present paint is in good condition then repaint with a paint made for bathroom ceilings or a good quality semi-gloss. I mix a small packet of mildewcide into a gallon of paint for bath ceilings. Keeps any mold from growing.

Handle the mildewcide carefully and read all directions.

Any paint store or home improvement store has the paint, primer and mildewcide.

Now on the other hand, if your ceiling is in good condition, has a sheen, it should be ok the way it is, unless you plan on spraying the ceiling with water.

If you run the pipe from the ceiling you don't need it waterproof.

Anyway, I installed one and I love it. Its like a shower in a warm rain.