Add/Edit Custom Fields Outside of wp-admin

Is it possible to edit post data outside of wp-admin? I’m just slightly confused on the best angle in which to attack this from.

I’m trying to add custom field data from an “admin only” pane on the post itself, without clicking through to wp-admin.

I’d (ideally, anyways) like to do it through ajax, even if I have to write my own functions…

I just don’t know where to point the POST (form post, not wp post) to, and what to include…