tbph, i doubt Germany would get involved in a civil war in their ally's territory other than providing some extra guns and ammo (the main reason both sides the ACW was on relatively equal footing at first was because the North was diverting their state-of-the-art guns and professional soldiers to the contemporary Indian Wars, iirc) and maybe providing some advisors like how Turtledove used Schlieffen in How Few Remain. i can't see dedicated German soldiers being fielded on behalf of the Union pretty much for the same reason that the UK didn't get involved--they don't want to get on the bad side of whoever wins in case the one they're supporting loses