Im not sure how much brick is seen around the window frame, if its only a little brick, you could paint it, although I'd prefer it be a smooth rendered finishor plaster like your other finished walls, because if there are three surfaces visible in there, none will be a feature, and all will look incohessive. To let the ply wall take the focus the plaster is perfect in contrast, so if absolutely necessary, pair. He brick white, although my recommendation is to gy rock and paint, just with he back wall in marine ply. Good luck, I'd love to see your finished result. By the wY, mentioned split battens to hang the ply, if you're unfamiliar, it's where timber battens are cut in half on the diagonal through section, and one half is fixed to the wall, the other the ply sheet, when they go together they become a secure locking fixing. Any handy person or tradesman will know how to do hat, without glueing or messing up the brick wall. It's quick, cheap and secure, and best of all can be removed without much effort to allow you flexibility in time.

My apologies for the awful delay on resolving this poll - it is REAL! ( I must have been caught up with Christmas lights and all the bon-bons etc) See the full project:

Also the house faces west.

what a fabulous looking house but i wonder how it would look if you rendered only the dark lower bricks and sills to match the render and keep the dark horizontal stripe above the windows.. a graphic artist should be able to make some images to help you see how it would look