I think that this argument could evolve into more of a discussion if certain stipulations were laid down as to the overall system one is talking about. Reading over the thread, I don't see where the table was properly set for what could actually be a pretty substantial back and forth, because I can see two sides to this coin fairly clearly, having been raised somewhat equally on classical and jazz harmony.
First I'd like to stipulate enharmonic equivalency for the purposes of this discussion, and 12-tone ET system (my rules for this post).
Next I'd like to stipulate that we are in the present tense--you can walk over to the piano and play the Moonlight or Brahms or Evans right now, and we aren't looking at history, we're listen to sounds, right now, at the piano. Okay, thanks.
There is a valid system behind what MR is saying, and it is the system I teach and developed over several decades. I believe it was first discovered by Erno Lendvai, who proported to find what he called the Axis system buried within Bartok and Kodaly's music. I'm not sure why it never took off--possibly because Lendvai developed a sort of "golden ratio fetish", and most of his book is (IMHO) wasted on some sacred geometry stuff that is, while interesting, more the stuff of stoned hippies listening to Debussy and seeing rainbows oozing from the speakers.
Getting back to the cool part of Lendvai's theory: Like Reimann, he assigns T, D, and S designations to the various diatonic chord degrees, but they are not all the same as Riemann's, and he assigns a T, S, or D designation to all 12 chromatic tones of the scale. He does away with mediant, submediant, supertonic, leading tone designations. Scale degrees I, bIII, bV, and VI are Tonic. Degrees IV, bVI, VII, and II are Subdominant. Degrees V, bVII, bII, and III are Dominant.
So to my point about setting the table, I think it's fair to say that within Lendvai's system MR has a point. I don't think he's made the point, but I think he has one, and is just not illuminating it very well. I have been guilty of the exact same thing many times--thinking that what I have written illustrates my point perfectly, only to read it over the next morning and go "what?".
So within the Axis system it is not only reasonable but absolutely correct to state that the N6 chord, being a bII, does indeed have a Dominant function, because that is merely stating the obvious--it's true by definition within the system. And it makes sense to jazz musicians who recognize the 4 ways that Dominant chords tend to resolve as being more equal. (Those 4 root motions being down P5 (authentic), down m2 (tritone sub), up M2 ("back door"), and down M3 (as illustrated by Schumann's Kinderscenen #1, bar 12, The Beatles "I Want To Hold You Hand", etc.).
So you're thinking "but it doesn't resolve to I, it resolves to V", and that is sometimes true, but N6 also quite often resolves to I6/4, in which case it makes perfect sense as a D to T progression (that then usually goes V : I, the more usual D to T progression.) What about the case where N6 resolves to V? We see it as a D to D tonality shift, but not a Functional shift.
There is a lot more to say about this system, but I will save it for another time.