Q: how to add keyword metadata to elements after upload

There is a way to batch add metadata, but it isn't iTunes-easy and you need to be have admin rights on your FCSvr to do it. Also, this is a manual process after each set of assets exist. Anything more automatic would require custom scripting.

You can use the Search Expired response to batch other responses. Create a new Search Expired response and set it to use the Custom 1 event type. Set the Timestamp Field to "Creation Date". Leave the Expiry Days empty. Below that you can filter based on the usual options or leave them all open to hit all your assets (up to you).

Next, create a Set Asset Metadata response and populate it with the value you want to apply to your assets. Set this response to run by Subscription based on the Custom 1 event type.

Finally, schedule your Search Expired response to run. When it does, it will fire what amounts to a dummy event (Custom 1) on the filter it was set for, triggering the Subscription you set up to listen for that event type. That fires the Set Asset Metadata response, and now all your assets are tagged with the desired default in the new field.

The Custom event types are there for tethering a Search Expired response to a Subscription, so unless you know what you are doing, avoid the other event types on a Search Expired response.

The official documentation does not really explain this very well, but it does indeed work. Once you get the desired result, you can disable the schedule for the Search Expired response.