Hmmmm - doesn't work very well that way on any of my Martins, because:-

a) The machines obstruct the placing of the PW Micro, i.e.there is insufficient room to place the PW Micro and rotate it so that you can read the (very small) display, because of the proximity of the D and G machines to the end of the headstock and the restricted space between them. In other words, the Micro doesn't fit very well in the restricted space at the back of the headstock.

b) When tuning the guitar, your hand blocks the view of the PW Micro.

And anyway, even if you place the tuner at the back of the headstock, the 'rear' arm of the clip still covers part of the headstock decal.

I have a D-18, an HD-28V and an OM-28 Marquis, and the above is true on all three.