Let's straighten out a bit of terminology to start with. Let's call the melodic degrees in question 6 and 7 when used in the major, and ♭6 and ♭7 when used unsharped in the minor. You're talking about the substitution of 6 for ♭6. Also, there's nary a major third to be seen in F♯-A-C - the chord is diminished, not augmented.
Normally F♯ is going to be a passing note in a rising scale segment from 5 to 1. However, it does present a possibility of being interpreted as part of a strong predominant. A diminished chord on F♯ would fall into this category fairly neatly (as shown in ex. a), as it would tend to suggest a truncated version of vii°7/V (ex. b) or V♭9/V (ex. c). So here, the diminished triad is standing in for the dominant of the dominant. I can't pull out an example in the literature off the top of my head, but the voice leading is so unexceptional that I don't doubt that it has been done.
Diminished chords are generally considered only moderately dissonant - they form, after all, part of the dominant seventh chord.
6 against ♭6 does show up in the literature, generally where ♭6 is falling to 5 and 6 is rising to 7. There is a rather baldfaced example in 2 voices in Domenico Scarlatti's Sonata in A minor K. 3:
Take a look at what is happening in ms. 17, 21 & 25. These all use 6 of the current key in the bass againgst a held ♭6 in the soprano. Yes, it is dissonant, probably more dissonant than it would be if mellowed by accompanying thirds, but it makes sense in the context of the voice leading. I haven't run across this cross relation used in exactly the way you posit, but I don't see anything inherently wrong with it, provided the voice leading works:
This uses your cross relation transposed by a fifth to fill out the Scarlatti example.