I've seen Brazil joining the Allies many times though, altough it's not historically accurate because usually Brazil needs to turn democratic first instead of Getúlio Vargas (Non-Aligned) joining the allies. On a related note, attacks against convoys from countries outside the war.
I've had Columbia, Uruguay, or Ecuador join allies over a war with Paraguay many times. At that point I just say screw it and join Comintern. WW2 turns into a 3-way over Columbia.
They will sometimes even do this if you declare on them as non-aligned nations like Bolivia or Guatemala. No consistency there though, sometimes they join a SA faction or nothing, despite 100% WT in every case.