IANAL, but by posting here you necessarily give HN an implicit license to distribute your comment, roughly equivalent to the legal boilerplate you'd find on YouTube or similar ("worldwide, non-exclusive, royalty-free license to redistribute, perform," etc). I don't think making it explicit with Creative Commons would be an "imposition". Creative Commons wouldn't give HN or anyone else ownership of your comment.
Opt-in wouldn't solve the problem for Hacker Monthly, as few people would bother. Opt-out would be fine. BTW, CC/NC precludes inclusion in Hacker Monthly as it is sold and contains advertisements.