I don't think it should be a problem as the sound is being projected from the head. As long as the drapes don't touch the heads or the shells you should be fine.
Legacy Forum Post
almost 10 years ago
If you have drums with outboard hardware and spiders outside the bowls, then draping material from the outboard supports is the way to go.
If you have center-supported timpani with the spiders inside the bowls, then it will be hard to drape anything without contacting the bowls. I'm not convinced that this would be a bad thing, but it may be. I have a feeling timpani bowls are heavy enough and rigid enough that damping them won't be noticeable. That should be easy enough to check with an old blanket.