Is it true that the meaning of あ可よろし is unknown?
No, it's not true.
According to the JA Wikipedia article, particularly the 絵柄【えがら】に関【かん】する注釈【ちゅうしゃく】 section, the etymology might not be fully pinned down:
- 「あかよろし」と書かれている。「の」のように見える2文字目は「可」の草書体に由来する変体仮名の「か」（） である。「あかよろし」とは「明らかに良い」という意味かという説もあるが定かではない。
Written as akayoroshi. The second character that looks like の (no) is actually the hentaigana for か (ka), derived from the cursive form of the character 可. One theory is that "akayoroshi" has the meaning of "akiraka ni yoi (clearly good)", but this is not certain.
So let's break this down.
Part 1: あか
Etymologically, we know that 明らか【あきらか】 comes from a root form ak- that underwent various inflectionary changes, of which aka is one. The Shogakukan Kokugo Dai Jiten Dictionary entry for あか【赤・紅・朱・緋】 notes:
- 「あか（明）」と同語源という。 ...
Apparently cognate with "aka (明)"...
Part 2: よろし
This is the root of modern adjective 宜しい【よろしい】. Shogakukan's etymology for this states:
A change from yorashi, or a derivative by similar means from yoru (寄る). In ancient times, yoshi indicated an affirmative judgment, whereas yoroshi was more reserved, implying a lesser valuation than yoshi, contrasting with the affirmative ashi and more reserved waroshi (warushi)
Drilling down, yorashi was an adjective with an original meaning indicating "something inviting approach, something you want to get closer to" (c.f. 寄る【よる】, "come near, draw near"), extending then to "good". The bit about affirmative versus reserved can be further explained as yoshi == "good", yoroshi == "not bad", waroshi == "not good", ashi == "bad".
(Note that this four-way breakdown is for ancient Japanese -- in modern Japanese, yoi == "good", yoroshii == "good [in polite speech]", warui == "bad", and ashi has almost disappeared, persisting only in certain set phrases like 善し【よし】悪し【あし】 "the good and the bad".)
Without further research to document this as a phrase used in older forms of Japanese, this remains only a theory, as noted in the JA Wikipedia article notes. I did search the online Man'yōshū at the University of Virginia's site, but I found zero hits where aka was immediately followed by yoroshi. But then the Man'yōshū is only one ancient text, leaving open the possibilities that any aka yoroshi phrase appeared later in the development of the language, or might have been dialectal, or might simply not have been used by the authors of the Man'yōshū.
All that said, given the etymologies of the parts, there appears to be ample room for this phrase to have been in existence originally as 明か宜し (aka yoroshi), without it necessarily being a corrupted or broken-down version of akiraka ni yoroshii.