It's important to recognize a few things about the purported 未然形 of ～い adjectives.
- All of the ～か～ forms are derived by contraction of ～く as the basic adverbial form + あ～ from あり (modern ある). This is sometimes called the カリ活用 (kari conjugation) pattern.
よく + あった → よかった
- As such, any 未然形 of an ～い adjective is actually the 未然形 of あり, which is あら.
Looking more specifically at the ～かろ form in your question:
- This isn't actually the 未然形 of anything. This is a later development, and is specific to the volitional.
The か derives from the same ～く + あ～ as above.
The ろう in the volitional ending is a contraction of らむ: む shifted to う, then the //a// in ら and the //u// combined into //ɔː// (like English awww) by the early 1600s, as evidenced in the 1603 Nippo Jisho. Then that //ɔː// sound merged with //oː// to produce modern ろう.
So the ろ in ～かろ is a weird kind of not-quite-stem.
If you can read enough Japanese to get around the page, the 古典日本語の形容詞の活用 (conjugation of adjectives in Classical Japanese) section of the 形容詞 page on the Japanese Wikipedia provides a conjugation stem chart. As you can see there, the 未然形 of the kari conjugation is から, not かろ.
The chart also shows the endings for the regular conjugation pattern, also called the ク活用 (ku conjugation) pattern. Educational grammars describe the ～く ending as the 未然形, since this is the conjugation stem used to generate the negative. However, I'd argue that there isn't a 未然形 for the ku conjugation pattern -- the ～く ending is simply an adverbial, and this can be used to modify pretty much any other verb or adjective -- including the negation adjective ない. I think this analysis is bolstered by the fact that the ～く ending cannot take various other auxiliaries that otherwise attach to the 未然形, such as ず or む. In fact, I've read in a couple places that the カリ活用 arose precisely because these auxiliaries couldn't attach directly to the adjective, requiring a verb with a proper 未然形 in the form of あり → あら.
Answers to core questions
Does the 未然形 of i-adjectives show up in today's Japanese?
Assuming you mean specifically the ～かろ ending, only rarely, as described in naruto's other post.
If you mean the ～から ending, then no, I can't say as I've ever encountered it outside of classical Japanese.
Bonus musings: would it be fair to say that 「高かろ is the 未然形 of 高い」 is an over-analysis based on 高かろう alone?
Yes, as above -- you're correct when you state: ...there is a sense in which you can consider 高から the true 未然形 of 高い.