My lights are dynohub powered, but there is no reason that you can't use a battery light on a similar mount.
I have my light under my bar bag mounted on the fork crown on one bike. I did a lot of bending of my bracket to get it just right, the bracket you see is not one you can buy off the shelf, but there are similar B&M mounts available. I did not have a long enough bolt to use through the fork crown, I used a threaded rod.
Maybe your bag is too low for that to work? On this bike I have my light lower and mounted on my right side canti brake mount. You do not have a canti post mount on your Surly, but maybe you have some other place where you can mount the light off to one side? The mount I used on this bike was the extra long B&M mount, I bent it to be more upright to raise my light up higher so that I had less shadow from the tire.
