Is there any way after 1865 that belligerent combatants from both sides of the ACW, & all colours- could've signed up en masse to serve as mercenaries in 'fre companies' in other conflicts in places like Latin America or Africa ?
I think if they went to South America it'd more likely be as filibusterers than mercenaries, but I guess it's possible.
I'm not nearly as sure about Africa. There wasn't much fighting going on in Africa as the Europeans did not yet have the capacity to penetrate the interior. When they did fight in Africa, they would probably regard it as an affront to their ability to wage war to need mercenaries - the era of mercenaries in Europe was decades gone.
There's only one man up to this job! (Of course he needs to survive past Sept. 1862 and return home to fight in the Civil War.)
I give you FREDERICK TOWNSEND WARD!!
Have him only injured in China. He returns home after being pushed aside by that glory hound Gordon. Once he recovers he's the type of guy who can't sit still so he, and some followers from China and the Philippines, form a regiment or two to fight for the Union. After the war he decides he's good at what he does and goes on to Mexico to kick out the Frenchies. Pretty soon he's the head of a de facto mercenary brigade.
While I can't remember any names, I do believe there were a few Union officers at least that retired and then saw service in the Ottoman Empire. There were a few, or one, that served on General Gordon's staff either in Equatorial Africa or the Sudan. Egypt and the Sudan were one set of destinations.