I'm sure Glenn will respond, but in general, you can put a layer of an open weave cloth (cotton, burlap, but not polyester) in front of just about anything and make very little difference to the acoustics. The simple test: if you can breathe through it normally, it's ok. (This test will get you odd looks in fabric stores. Ask me how I know this.)
Some tighter cloths might conceivably reflect high frequencies, so I'd avoid recovering traps placed at first reflection points, if imaging is very important to you. Or I'd use burlap there - dyed burlap can be made pretty.