Thanks for the heads up. The light bar, unlike the with the mounting you show, will be below the roof line. (It would have been easy to use the supplied mounts to stick it up there higher!)
The light bar end caps are cast aluminum and threaded for the mounting bolt, looks robust enough to torque tightly, so unlikely they'll come loose - on the other hand, the threaded holes are not all that deep, so screw engagement length is less than half an inch. If they have a tendency to loosen, a bit of Loctite or Vibra-Tite thread locker should do the deed. Also, I'll be machining some black Delrin spacers for each side. The elasticity will give the bar end serrations more purchase - to help keep the bar from rotating, as well as to keep tension on the mounting threads (kinda like a lock washer would). The curved bar does cantilever some weight forward of the attach bolts, a straight bar wouldn't be so bad in that regard.
Searched but couldn't find anything on a "threaded rod fix", could you provide more info or a link please?