We have many small mongodb servers. The request is to have online copies (slaves) of all databases in one place/server

There is option in mongod:
--slave; --source [server:port]

This option could be extended with possibility to use:
--source1 [server:port],--source1db [db], --source1collection [coll] etc etc etc
to allow read oplog from many Master servers and REPLAY them on the target server.
This solution would allow customers to build ONE archive server with ONE mongod running and holding online copies (slaves, rs) from many many databases/collections.

thanks \marian