[ownedbycats]

Based on something I actually tried:

Set Generate Cover > Contents > Metadata (Not Saved) fields to either accept literal input or templates, like the custom text field. I can just paste in the subtitle template when needed.

More thoughts, but just for convenience:

Have 'Save' and 'Load' buttons. Save would save the values (presumably templates) to the config (or cover template perhaps). 'Load' would load them in. Continue to auto-fill, but 'load' could be used to override when needed.