While it is possible to use an embedded field as a shard key, it is not possible to define tag ranges for such a key.

For example, I have set up the field "_id.ts", a timestamp, as a shard key, but defining a tag range for it

sh.addTagRange ("ncdc.data", { "_id.ts" : MinKey }, { "_id.ts" : ISODate("2012-12-01T00:00:00") }, "archive" )

results in the error "can't have . in field names [_id.ts] at src/mongo/shell/collection.js:145". This is probably an implicit restriction due to the way tag ranges are specified, but it is still unfortunate and should be possible.