The idea of JC having kids has been around a lot longer than The Da VInci Code. Hell, it was a major plot point in Dogma, which was released 4 years before the Da Vinci Code book came out.

The theory about Christ's descendants goes back at least as far as the 1982 release of "Holy Blood, Holy Grail" by Baigent, Leigh, and Lincoln. The same theory pops up in the Preacher comics in the '90s, and Dogma. I think it's almost impossible that Butcher isn't well aware of the other takes on the theory, and if bigfoot is fair game, so is this.

OTOH, much is made of the power of belief in DF, and the belief that kings have divine power pops up in a lot of cultures. That and the right attitude might be enough to make someone a good Knight of the Cross.