It's probably one of those things that nobody has ever done before, and the administration is saying "no" just because "we've never done it that way". I'm sure your idea is too forward thinking for some principals to grasp.
I fail to see why such a suggestion would meet with any resistance. The child is in class with the same teacher all day, so it's not like she never interacts with your child. I doubt she'll be saying anything in front of your son that he hasn't heard before. Seems kind of odd to me.
Possibly, particularly because there are no major issues being discussed, the teacher is just looking at it as a quick, administrative thing? You know....she has to check it off in her book that she met with you, but she didn't really plan on it being a big whoopie-doo?
Honestly, I would just show up with him anyway. When they say "it's against policy", tell them you want to see the policy. Since I doubt it's in writing anywhere, I would say you'll be at checkmate within a few minutes.