Lets say im in c major and i play a standard 1-6-4-5-1 progression. But on the second time around i play 1-6-4-#5-5-1. The #5 chord is obviously an Ab flat major/ G# major, and i know many scales can be played on top of it. My question is, if i were to use the diatonic c major scale over this chord, either with or without hitting its chord tones, what would be the name of that technique? Thank you.
More "academic" music theorists often refer to this more broadly as stratification (or layering).
This occurs when a piece is composed in "layers" that are stacked vertically on top of each other. (Think of different geological strata.) The outcome is that simultaneous musical elements (scales, chords, etc.) can seem to have little or no relationship between each other, as in your example of a diatonic scale occurring simultaneously with a non-diatonic harmony.
It's just called a chromaticism or chromatic harmony.
Chromaticism is a compositional technique interspersing the primary diatonic pitches and chords with other pitches of the chromatic scale. Chromaticism is in contrast or addition to tonality or diatonicism (the major and minor scales). Chromatic elements are considered "elaborations of or substitutions for diatonic scale members".
A quick note that the use of chromaticism does not mean you are tonally shifting away from diatonic harmony or changing keys and in fact in most classical cases (like secondary dominants, Augmented 6ths, ect) you are reinforcing your tonic in a different flavor.
For example, in the progression you list, the I - vi - IV - VI♭ - V - I, the VI♭ pulls you to V which pulls you to I and this extra tension and resolution helps typical tonal ideas. Specifically this will most likely be referred to as modal borrowing since the VI♭ chord can be found in other modes of C (specifically Aeolian or Phrygian) , however it's still chromatic in nature which is the more prevailing effect.