The indexes on config.chunks are

{ns: 1, min: 1}, {ns: 1, shard: 1, min: 1}, {ns: 1, lastmod: 1}

(defined here).

This will probably touch a good bit of code. Backwards compatibility must be maintained, so we'll have to continue writing it, which should be easy because _id is the concatenation of the namespace and min key fields.