Well a Ghoul is already the scion of a human/uber-ghoul relationship (if you can call it that). As such, I would so that it's possible to have a ghoul/human half-breed (quarter-breed really).
Of course, if you're talking about the real world figure rather than the myth that's been built up around her you could probably call her a ghoul even with a standard Pure Mortal template. Alternatively she could be a WCV from house Skavis?
That was my thought on it, since humans and ghouls were obviously compatible at one point. It could still hold true, and would also explain how such an entity could keep their consumption of human flesh down to a concealable limit (not a full ghoul, not a full apetite).
One of the most Holy People of the 20th Century a ghoul? The Living Saint she was called when she was alive a GHOUL?! Excuse me?! As a Ghoul there is no way she could have entered some of the Holiest Grounds in the world without being burnt to a crisp!? I am not just talking Christian Holy Places but Hindu as well. She also lived in a Convent and went to Church. She would not be able to do that with that design in the slightest.
There is no way Mother Theresa was a Ghoul! She was a True Believer dear God study the woman.
As DFJunkie points out, there is no proof ghouls are allergic to holy stuff. I cannot remember any such proof anywhere in the series, though I could be wrong and would welcome page references. And as for studying her...I have. I'd be happy to get into that debate with you, if extended riffs involving religion weren't against the rules of the forum. Suffice it to say that I have come to very different conclusions.
(And as for True Believer, I can argue that, too; read what some of her personal letters, preserved after her death, contain.)
DF Junkie, I was thinking of using her as a mix of Major Power and Antagonist. I liked the crossover between the Catholic Church and the Sabbat in the Old World of Darkness Material, and I was thinking that this could be very fertile ground for a variety of reasons; the emotions, the imagery...my main allies are actually intended to be the poor most mistreated by those in power, the ones who actually need the help and yet never actually receive the resources and money supposedly earmarked for them. 'Prayer of the Refugee' by Rise Against is some of my inspiration there.