I'd be able to very quickly tag a bunch of photos for later processing.
The Tags feature in Finder1 already has a keyboard shortcut which can be activated whilst in Quick Look. Press ⌘⇧T to set a tag on the currently selected file.
To set a custom shortcut…
- Open System Preferences → Keyboard → Shortcuts → App Shortcuts.
- Click the + button to add a new shortcut.
- Set the Application to Finder.app, and the Menu Title to Tags….
- Set your keyboard shortcut and press Add.
1 As this is a feature of Finder, not Quick Look, the modal popup may be behind the Quick Look window.