See About Less Wrong:

Copyright is retained by each author, but we reserve the non-exclusive right to move, archive, or otherwise reprint posts and comments.

Also,

We reserve the right for moderators to change contributed posts or comments to fix HTML problems or other misfeatures.

One of these days the Creative Commons people will get around to writing a variant that takes into account the moderator's need to edit malformed/misspelled [X]HTML.

To answer your question: yes, you can license it cc-by-nc-sa, but it might conflict unhelpfully with the moderators' reserved rights. IANAL, but a cursory google shows the matter hasn't noticeably been tested in court. It might be possible that if a moderator modified your post to correct some spelling or markup, they'd be infringing upon the cc license.