And unless AMD approves Nvidia, that's not gonna happen. Mantle isn't open-source or anything, and even if the AMD has said that they *could* allow Nvidia to use it, I somehow doubt that they will. And if they would, they definitely wouldn't allow Nvidia to integrate any Nvidia specific features/optimizations etc.
Considering how hard they're pushing the "other vendors can choose to implement it too!" angle at their conference right now, I'd be surprised if they tried anything like that.