Yeah, I think the rules are fairly clear here. If you have a gun with 360 degree arc (e.g. turret or pintle mounted) then why not a searchlight?
Since you normally need LOS to fire, then you'll also need it for the searchlight. As far as I know, nothing stops you illuminating a target having fired an indirect barrage though! (Do any barrage weapons have searchlight options? Has that been FAQed?)