addTagRange doesn't work with embedded shard key

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).