I know the 7D meters based on the center. There is even a circle that appears when you switch to spot metering. So, yes, it will meter off a person's shirt, etc (especially in portrait orientation). I recently shot a big band rehearsal, and as they were in casual dress, shirt colors varied. Shutter speeds were all over the place. I would spot meter on your subject's face (or preferred body part
) in Av, note the shutter speed, and switch to Manual. Give yourself a high enough SS so that you can drop a bit for darker portions of the stage/scene, and increase it for hot spots, etc.
Hop this helps...