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.