How does "thought-provoking dissonance" in Zen Buddhist dialectics differ from "wrong speech"? Zen question-and-answers and talks like the ones described here don't seem like commonly accepted 'right speech'; yet, they are believed or intended to trigger spiritual progress.

In what way do such Zen interactions deliver benefits? For example, are they meant to be thought-provoking dissonance, discomfort and anger, and thereby cause dissatisfaction with habitually-held positions, and thereby create scope for spiritual progress?

Have the teachers specified (in any writings or talks) how to distinguish such dissonance-causing dialectics from "wrong speech" which causes hindrance in spiritual progress and delivers no spiritual benefits to listeners/readers?
First off, I think it's important to understand that Zen isn't something that should be greased up and crammed into the strictures of traditional Buddhist teachings.  Zen makes no overt attempt to follow the path as laid out in the Sutta Pitaka (i.e. in the Samanaphala Sutta) nor does it conform to the gradual training as described in texts like the Vissuddhimagga.  Topics like right action, sila, and virtue, while undoubtedly present in the Zen tradition, are meant to be arrived at through direct insight.  They are realities to be discovered, not guidelines to be followed during the initial stages of practice.   

The particular quotes that you are referring to are from two koans.  Specifically, they come from cases 1 and 21 from the koan collection entitled the Mumokan.  In Zen practice, koans are designed to point at some subtle experience of reality (i.e. big mind) that is often missed by our discursive and discriminating smaller minds.  They are not designed to be "thought provoking", nor do they try to be necessarily humorous, provoke anger, or instill discomfort.  They don't even try to cause dissonance.  For these reasons, they can often sound completely nonsensical.  In fact, koans are "none-sense" in the purest definition of the word.  No amount of reasoning will help you arrive at the experience they are pointing to.

That being said, koans are surgically precise.  They aren't just "right speech" they are "exact speech".  The experiences that a koan points to are ridiculously specific and, in the hands of a master teacher, can be used to great affect in awakening a student.  A good teacher knows exactly when to introduce a certain koan.  Like someone playing a game of Jenga, he knows just what block to pull out in order to get a student's whole mental edifice to come toppling down.  

All koans seek to awaken the mind.   Not a single word of them is tossed off haphazardly.  They are skillful means - upaya - but are functionally quite distinct from anything you might find in the Theravadan tradition.