What are the sources for reciting Kaddish for a non-Jew?

Sefer Chassidim (Rabbi Yehuda haChassid, 12th century Germany) who said that one may pray for the soul of a gentile who saved Jews in times of persecution and crisis. Later authorities expanded this ruling for a ger (convert) to say kaddish for his non Jewish parents. Apparently there is no prohibition of saying kaddish for a non Jew, as the kaddish prayer is always acceptable, but the custom seems to have been limited. Rav Ovadiah Yosef (Yechaveh Da'at Vol 6 Responsa 60) rules that a ger may say kaddish for his parents and brings a variety of sources. Noteworthy is that the Sefer Chassidim quotes the Yerushalmi (Megila Ch. 3 Par. 7) that one must bless Harbonah who gave suggestion to hang Haman.