As I understand it, mongos on 2.2 supports specifying read preferences, which are applied when the query is proxied to the underlying mongods (feel free to correct me if that's wrong).

I'm pretty sure I've successfully passed a read preference to an individual query to a ShardedConnection, but it doesn't look like ShardedConnection supports specifying a connection-level read pref like ReplSetConnection does.

It seems like it should, but I'm not entirely sure I understand how the pieces fit together. Is that the sort of thing that makes sense?