@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 ii
or iv
.
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.