Thanks for the suggestion; however, I believe that feature is – by definition – used to create a new topic and not a reply. In fact, it’s the Compose a new pre-filled topic via URL feature that inspired this feature request – i.e., if we can pre-fill the composer for a new topic, then perhaps we could do it for a reply to a topic.
I would imagine the Reply Template feature would be implemented as a hook on the reply button. When a user hits a reply to topic (i.e., not a reply to post) button, it would scan the topic for a [template][/template] element and, if found, would pre-fill the composer with its contents.