Sorry, that pesky job of mine has kept me busy.
I am not sure that what you are asking for can be done. Actually, it sort of can, but with a lot of work.
I don't know how to specify regimes, or peoples in random games. The mechanics of random games and maps are in the rulevar section (under the Setng button) "Random Map Instructions". Random games are generated using six possible regimes and peoples, these are labeled A, B, C, D, E, and F. You specify the the string list for officers, the extra graphic, the string list for regime names, and the people. The problem is that it seems to use all six, and you can't say which ones you want. I wish you could because it would speed up my random games and I wouldn't reject some. I tried to specify just two, by putting -1 in the entry for the rest. No dice. It CTDs. So, in the current setup, it isn't possible, as far as I can tell. Repeating them half Japanese, half American would work sometimes if you had only two regimes. But, sometimes, it would have Japanese vs Japanese and American vs American. So, it could sort of work, you would just have to reject the games that were ridiculous.
Now, that said, I think it would be possible to make an event that would transform a two regime game into what you want. But, it would require several things. The cities have to be renamed, the regimes renamed, and peoples. I think these are all doable. Then you should remove all the units placed, and then cycle over the cities to place new units for the regimes. It is a bit tricky, but I think quite doable. I haven't tried, but I think all the functions are there. It would take a few hours to build. One issue that you might not like is that with a ANewDawn, you will have to use the pictures for the British officers.
One thing I tried, and I don't believe is doable is to invert the full map. There are some aspects of the map that can't be placed.