Like others have mentioned you can use a spare chain through the seat rails and frame. It may be a cheaper solution than chainging the the quick release bolt or collar especially if you have it lying around.

The saddle leash by bikeregistry or something similar is also an alternative.

Personally I would like both as most people I don't think actually want the seat post but the saddle. A saddle is usually more expensive to replace (depending on the saddle) than the seat post.