If the shard key is defined on an embedded field there is no way to call
sh.addTagRange on that shard key.

My example was tweets sharded on "user.screen_name", created_at:
twitter.tweets2 chunks:
shard0001 1
{ "user.screen_name" :

{ $minKey : 1 }

, "created_at" :

{ $minKey : 1 }

} -->> { "user.screen_name" :

{ $maxKey : 1 }

, "created_at" :

{ $maxKey : 1 }

} on : shard0001 Timestamp(1000, 0)

Trying to addTagRange basically always gave:

sh.addTagRange("twitter.tweets2",

{"user.screen_name":"A"}

,

{"user.screen_name":"z"}

, "data")
Mon Sep 10 14:30:38 uncaught exception: can't have . in field names [user.screen_name]

Trying to specify it as {"user":{screen_name:value}} is accepted but it doesn't have any effect (presumably it doesn't match the actual shard key).