Installing corrugated metal in bathroom
I don't consider it any more of a concern than wood wainscot or a vanity that's not hermetically sealed to the wall. I take measures to prevent undue splashing in my bathrooms, such as using a shower curtain and teaching my kids proper behavior. With this approach you can have confidence that your nice, unique metal wainscot won't result in the fall of civilization. 

I'd run a wooden cap as shown in the photo, and if you're concerned about water running down behind, seal against the wall with a fine bead of clear silicone caulk. It'll take on the colors of the wall and trim and be nearly invisible.