When voting to close a question, there's no "low quality" option
There is, but only if the question isn't already in the Close Votes review queue. Proof. Its use however is debatable, since the outcome of a question in the Low Quality Posts review queue is to close it. It's better to use one of the standard close reasons.
the "low quality" flag is intended for moderators
Well, not really. This is what it says:
This answer has severe formatting or content problems. This answer is unlikely to be salvageable through editing, and might need to be removed.
But you're right, in a sense:
More specifically how do I mark a post as low quality, but not intending moderator attention?
Flagging a post as Very Low Quality (or Not An Answer) will put the post in the Low Quality Posts review queue (if it didn't already get a review there) and simultaneously puts it into the moderator flag queue. There is really no other option, other than simply downvoting the post to indicate it's a poor quality post.