I have a forum with 3 custom fields. One is a URL single line text. It does not show up in the quick post. It does on an edit after the post. It also show up when you click the new post button.
Reading this, I have become curious for the reasoning behind this. We use custom fields in our classifieds forum and two of these custom fields are dependent on the values of other custom fields.
One field is delivery method:
Pick up only
Mail only
Both pick up and mail
Location: only needed when pick up is chosen.
Now location isn't displayed as an option when creating a quick thread in classifieds.