I wrote a GUI for git-annex metadata in Python: git-annex-metadata-gui. It shows the files that are in the current branch (only those in the annex) in the respective folder hierarchy. The keys that are in the repository, but not in the current branch are also shown in another tab. You can view, edit or remove fields for individual files with support for multiple values for fields. There is a file preview for image and text files as well. I uploaded some screenshots in the repository to show it in action.
While making it, I decided to move the git-annex calls into its own Python package, which became git-annex-adapter.
I hope these can be useful to someone other than myself as well.