Covering RPG Skyline with fabric

Most likely not by much but could compromise there diffusion quality with fabric. I would contact RPG they have a large line of possible fabric choices and should guide much better then taking pot luck.

Of course it will work perfectly. Just use something similar to speaker grills - as near enough acoustically transparent as you can find.

Guilford of Maine is well known for acoustic fabrics. You can find a dealer or try direct - a web search will get you there. There fabric is used for a wide variety of acoustic treatments as well as speaker covers.