Sorry, I meant to reply to this earlier. I'd make it just a link that goes to a form for you to compose your reply (with lots of room to type), with the author's name prepopulated and "re [id://...]" prepopulated. I wouldn't put it in a nodelet at all as that can lead people to replying to a parent node when they meant to reply to some deeper reply that they were reading on that page. It'd be even better if the node content is displayed underneath the reply form.
It'd be nice to have a good term for "private reply to node via /msg" as using "reply" even as part of the term will be confusing.