There "can" be a relation with folder rules. One way to achieve such a functionality would be to code a folder rule (using a script action) which reacts on the event that metadata (via aspects) is set on the folder, and then the script runs through all existing documents in that folder to copy the same metadata to those documents. Of course you would have to be careful to only copy the metadata that was set / you actually want, and rules unfortunately cannot determine what the metadata that was just set was - they only are able to access the full state as it now is.
Another way to achieve this kind of functionality would be to develop a repository-tier action which can be invoked via a form on a folder from Share / whatever client you are planning to use, and have that propagate the data entered into the form to all existing content nodes, without the need to actually set and duplicate the metadata on the folder first.
