Actually, the idea of Doc Savage having a non-caucasian mother has been around since at least 1973, when Philip Jose Farmer mentioned it in his book Doc Savage: His Apocalyptic Life. Farmer was a fan and his book combines stuff from the original pulps with his own inventions, so in that sense it was "fannish speculation" but at this late date anything anyone says one way or the other can only ever be fan speculation, as Lester Dent is no longer around to ask.
I have to disagree with your previous respondent on one point: I don't believe that Dent would have ruled out the possibility of Doc's mother being a native of Hidalgo, or be horrified at the thought of interracial parentage. Quite the opposite; he was a very progressive guy. But there would have been a lot of trouble trying to print such a story in that era -- which, come to think of it, may be precisely why Dent never spelled out the identity of Doc's mother. If he wanted her to be another race but knew it would never get past the editor, is it unreasonable to think the author might just keep it to himself?