@Dom's recent edit brought me here.
I'll just add this to @Basstickler's answer which I think gets to the point that Neapolitan chords are a concept within the major/minor system.
I think of Neapolitan - and augmented sixth - chords as first and foremost chromatically altered diatonic chords. The colorful effect of these chords is made apparent by their alteration from diatonic subdominants
You could make an analogy with language. A phrase like 'Trompe-l'œil' sounds very intellectual in English, but in French it just means 'trick the eye.' In the context of a borrowed, foreign word it sounds special, in the context of the native language it is just a few simple words.
Why use a borrowed French phrase in English to convey an idea about tricking the eye? Well, the borrowed phrase is used for a very specific kind of optical trick: when a painting is so realistic it tricks the viewer into believing they are seeing a real scene. We say 'trompe-l'œil' convey they specific meaning not just any optical illusion.
So, in Phrygian, a major triad build on the second degree of the mode - a half step above the tonic - is nothing more than the major triad on the second degree. That is the native root and chord quality in Phrygian.
But in a minor key a major chord on a lowered second degree is something special. Of the common chromatic chords - Neapolitan, augmented sixths, secondary dominants - it is the only one whose root is altered by lowering. It's so special we give it a special name to underscore its unique and foreign character: Neapolitan.
In a mode where the second degree was natively a minor second, I think the label Neapolitan isn't meaningful.