I am going to preallocate daily indices ahead of time and move any poorly placed shards. That won't work for everyone, but we have very predictable shard sizes so automation can decide whether it's a poor placement in advance. Moving an empty shard is very fast. 
This "just" leaves us with optimal disaster recovery shard placement. Hopefully when that happens we'll be on the version of Elasticsearch that has best-fit shard allocation taking free disk into account!
Ken