I have read that Jews recited kaddish on behalf of Abraham Lincoln after he was assassinated. What is the halachic basis for this and has kaddish ever been said en-masse on behalf of other non-Jews?
Ha-Rav Ovadiah Yosef rules that it is permissible and proper to recite Kaddish (Shut Yechaveh Da’at 6:60).
Rabbi Ephraim Oshry in Responsa from the Holocaust #85 allowed survivors to say Kaddish for the gentile woman who had hid them in her house. With justification, it seems allowed.
See there for more sources.
Rav Eliyashiv in Ashrei Haish 91:1 holds that a convert may say kaddish for his father who passed away.