My question is somewhat simple: is there an honorific for particularly righteous, deceased non-Jews? Related: is זכרונו לברכה ever used for non-Jews? I don't recall ever having seen this.
EDIT: I mean honorifics appended to the end of the name.
By looking at the source we can understand the answer. OB"M or ZT"L comes from the pasuk (Mishlei 10:7) "The mention of a righteous man is for a blessing" Assuming that only a Jew can achieve the level of "Tzadik" that Mishlei is refering to, it seems that the honorific would only apply to Jews. [Non Jews can achieve the level of Pious Among the Nations at most see http://en.wikipedia.org/wiki/Righteous_gentiles#cite_note-SSMH27-4 ]
NOTE: If you are writing/speaking in a forum where the honorific is used for non-jews then I believe that it should be used for derech eretz purposes.