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
about 8 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.