I was building a small dashboard for mongodb which connects to a mongos and lists the current status of all servers in the network.
While working I found there was no way from my connection to the mongos to query the replication status, health, or stats from shards or individual servers.
I ended up having to parse the host for the shard and make connections to the individual shards directly to check their health.
I think it would be better to have a way to send commands through mongos to reduce the nodes that need to be opened up to make this kind of feature.