Do you want rails under and over the whole length or just the end? I am not aware of any that do the full 12 and 6, but some that do full 12. Also there are some out there that do 12 and 6 but at the end (like the YHM Todd Jarret competition rail). Or just the 12 at the end like the ALG Defense EMR.
You could always go with the Aero monolithic upper, I think it has rails only at the 6 and 12 position.