http://docs.mongodb.org/manual/reference/mongooplog/ claims that "mongooplog" supports --db as such: "Use the --db option to specify a database for mongooplog to write data to. If you do not specify a DB, mongooplog will apply operations that apply to all databases that appear in the oplog. Use this option to migrate a smaller subset of your data.", and similarly for --collection

Empirically, and based on reading source, mongooplog accepts a --db option, but does not do anything with it – it will continue to apply ops applying to any database.