The stairs and at least one plank leading from one wall to another are indeed only there in the class quest instance. This means only Captains will ever find them, and then only when running the class quest.
Also, I prefer using a horse for the last standard. However, I recently discovered that a horse is not necessary for reaching it, having jumped on foot and arrived safely. (This may be a recent change, as that did not work for my first Captain 6+ months ago, but it did work for my second one on a different server about a month ago.)
Gimli's First Law of "Let's go!"
"Certainty of death? Small chance of success? What are we waiting for?"