Having been to Japan several times… and actually studying Japanese studies… I can see why you're saying this but it's very superficial. None of the southern "essence" that caused the split & war is there in Japan at this point. Or do you mean it's as if the South seceded, then proceeded to lose anyway due their policies being a natural disadvantage, and then developed, out-northing the North?

What is characteristic of "the American South" here?

You may think that because of the nationalism, but that's a superficial similarity. Outside western europeans, almost everyone is nationalistic, so that's not a useful characteristic in comparing societies. Aside from that, the American south is more like the Arab world/India--disorderly, combative, face-saving culture, and food-, family-, and faith-centric. The Japanese are much more like Mormons.

The “became technologically advanced” part of this hypothetical is doing a lot of heavy lifting.

Yes and no.

The biggest historical commonality between the American South and Japan - or, at least, the Tokugawa shogunate that was ruling around the same time as the South - was that both of them were at one point ruled by a cabal of essentially feudal lords. There is a lot of convergent social evolution that happens when your country's economic elite all make their money by owning land - lots of strange social titles, honorifics start floating around, people start talking around problems, and the like. However, this comparison is only skin deep, and there's plenty of things the Tokugawas did that have no parallel, or even worked backwards, to what happened in the historically contemporaneous American south:

- Sakoku (the policy of keeping Japan completely closed to trade and immigration).

- While America generally has had a long streak of veneration of the police, they never reinvented anything close to the samurai in terms of being a hereditary warrior class with a tax stipend.

- Likewise, the American South did not have a caste system; Japan did (and so did France?!?)

This makes more sense when you remember that the Tokugawa shogunate itself is the time reversal of the American South: the Tokugawas were born out of a long civil war and ended with a foreign invasion (from America) while the American South was born from foreign invasion (of Americans) and ended with a civil war.

The American South is also, interestingly enough, a mirror of Japan in other dimensions as well. The reason why the Tokugawas shut the country's borders for hundreds of years was because they understood that Christian missionaries were a front for a larger invasion force that would pillage the country's resources and abduct people to become slaves in other countries. Of course, the South was buying slaves abducted in other countries in massive quantities.

Going back to the convergent social evolution I mentioned before, you can see some of this in Japan today. Kyoto is notorious for doing exactly the same kind of backhanded compliment bullshit the South loves doing. But even then, that's one part of Japan, that was formerly the capital city. One stop on the Shinkansen takes you to Osaka where people are about as open and outgoing as Americans are.