I'm trying to wire a light above a door that has a 2X6 header. The header has a gap between the 2 boards (there is no 1/2 inch plywood). Is it okay to run NM 12-2 wire through the gap in the header? Does this violate any codes?

I am assuming this area will be totally enclosed when finished and there will be no movement, right? You should be fine with this but I would slide in a piece of 1/2" ply to insure the headers never shift and try to come together.