This was the setup I had on a bike I sold. Not exactly a Surly rack, but similiar to what you're talking about. It's a Planet Bike Sport Spot mounted to a Tail Light Bracket. Believe it or not, that little Sport Spot put out good light for its' size and price.
I have the Surly Nice rack on the front with a light. I just ran a pice of 3/4 inch plastic pipe behind the front upper curve and used to two standard pipe clamps to secure it to the cross bar of the rack. This puts it a little lower than the platform mounted braze on, and the lamp just off to the right side of the rack. This allows stuff to protrude over the front a little bit without interfearing with the lamp mounting. I am using a Cat Eye EL530 to commute in the mornings and it works great. This setup will also allow any handle bar mount to be used. I could have made the pipe longer and had a light on each side of the rack. Sorry no pic.