A website promoting a religion other than Judaism has a statement claiming that a Jewish source exists claiming something that appears to conveniently support their own religion (and for which I can find no actual support in Judaism). I contacted that site and asked them for a citation to a Jewish source for the assertion (e.g. a specific Rabbi, Talmud passage, etc.), and they replied (!), stating that they did not know the specific Jewish source and that their source was a claim by one of their own leaders that such a Jewish source existed. That leader is long dead and unavailable for comment.
Would it be appropriate to ask on this site about whether such a source exists, even if it represents a minority view? How can I phrase such a question? E.g. "The XYZ Temple of the Holy Idol of Saturday Shellfish claims that many Jews at the time of King David believed that it was ok to do X and Y if Z condition was met. All of the Jewish sources I can find indicate that X and Y have always been forbidden even if Z - is there a Jewish source claiming otherwise?"
I know that one of the close reasons here is "Questions about comparative religion, and questions about what others have written about Judaism, are off-topic on Mi Yodeya. This includes any question that requires of its answerers any knowledge of a religion besides Judaism.", but I'm not sure if it really applies in this case as the question is not about the claim on the non-Jewish site (e.g. the nature of the claim, how it should be interpreted, when it was first published in the literature of that religion, etc.), but is only using the claim to ask a question about Judaism and provide support as to why the question is interesting, relevant, or worth answering.
Yes, I have thought about simply asking the question - "Does a Jewish source exist claiming X?" but I am afraid that such a question would be poorly-received and gather comments asking me why I would ask such a weirdly specific question and what would make me believe X could possibly be the case to begin with.