I got an email back from Jeppe at Evil Twin yesterday. He said that all the variants of Femme Fatale are brewed with 100% Brett C., so it is a different yeast than the WLP644. I don't know if its a proprietary strain of Brett C., but I'm definitely going to play around with it a bit, and try a 100% ferment with it soon.
I have a healthy starter going right now, and I'd be happy to swap some -- let me know what you were thinking, Kyle!