Perhaps you could add a mention of the license on the project page and/or at the top of the file?

If I'm understanding you right don't have a former license yet. You could use something like MIT, Apache 2.0, CC0, GPL, LGPL or WTFPL, that would fit your comment.

I think MIT or Apache 2.0 would be the safest choices (if you don't have a preference for GNU/copyleft licenses).

I'm afraid a HN comment won't hold up in court as proof I have a license to use a work.