Why not just add a hatch to the box you considerd so you can open the box and access the valve? It shouldn't be too hard to find a prebuilt valve box that looks decent in your bathroom. Most I know are for outdoor applications but no reason why they can't be used indoors. And building it yourself also shouldn't be too hard.

To make it look even better you could also consider putting a removable shelf on top of the valve box. That way it's more hidden and you have a spot to put soaps or other bathroom essentials.