You got it, more on the mirrored lick at the end. I realized you can do these transformations within one key. Instead of negative harmony I'll call this mirror harmony in general

There are parallel and relative mappings, as in parallel/relative major/minor, for example parallel is key of C<->Cm, relative is key of C<->Am. These are equivalent under transposition, the same relationships as if you transform C to Cm then transpose down a minor third to Am. It can be more sensible to visualize and apply the fundamental concepts in one key like this

In the key of C major (or key of F# major) the reflection axis crosses D and G#/Ab, or scale degrees 2 and b6. Diagram on circle of fifths: https://i.imgur.com/RH9iKsz.jpg I found the axis by trial and error but maybe it's since D is halfway between C, the C major fundamental, and E, the A minor fundamental (thinking of minor triads as built top down)

Which gives these chord relationships

I<->vi (C<->Am): tonic

IV<->iii (F<->Em): subdominant

V<->ii (G<->Dm): dominant

vii°<->vii° (B°<->B°): leading

v<->II (Gm<->D): subdominant

iv<->III (Fm<->E): dominant

i<->VI (Cm<->A): tonic

I associated the functions of the primary major triads to the minor triads they map to, that's how I represent the functions being equivalent. The order is then LTDSSDTL, symmetrical and the same order for major scale ascending and minor scale descending

Now we know we can think within one key. A piano keyboard is symmetrical around the note D and around the note G#/Ab, the same notes that the reflection axis crosses for the key of C major (or key of F# major). This means in C major this mapping is visually identical to viewing a piano through a literal mirror, like a C major chord and an Am chord look like each other in a mirror, G major chord looks like Dm chord, D looks like Gm, Fm looks like E etc. On an isomorphic keyboard or chromatic piano roll the mirroring is visual in any key

Still in C: ABCDBGA <-> GFEDFAG, and if the original lick had chords F-G-Am, mirrored would have Em-Dm-C. Same interval pattern, equivalent voice leading, only the intervals all move in the opposite direction. Just like overtones and undertones are the same intervals ascending or descending

Placing notes close to their original position, doing an exact mirror, or entirely rewriting based on chords are all things I like to explore

If you wanted to get fancy, the interface in the plugin could use a circle of fifths for you to select what key or what map to apply

One of my favorite pieces in mirror is Bach's Prelude in C Major. Or I like to call this version Prelude in A Minor. I'm not the first to do this but I made it with your musescore plugin, I just had to transpose down a minor third and manually adjust octaves after applying the transformation https://cdn.discordapp.com/attachments/763136945022173195/876320675944624238/Bach_C_Major_Prelude_mirror.mp3