I would like to do the opposite, and use existing Metadata to pre-fill Workflow Forms.
That’s not possible but it is a great idea. Following up on that.
One other idea. Is there a way to have a workflow bring up the Metadata interface? Then the user could edit it directly.
It is no possible. Because they are different apps. One way to accomplish this would be to allow the data entry on the workflow form field and then parse it and update the metadata using the workflow action as the tutorial shows.
It is an intriguing UI flow. We have been experimenting with a feature that allows having two simultaneous Mayan view open at the same time (Mix documents - #2 by roberto.rosario). This might be something that could take advantage of that. This is still an experiment and not yet ready for inclusion. Is just one of the many experiments we run to explore the future of document management.