I'm puzzled. Are you talking about a 'pylon' type sign? Or a sign of some other configuration out 'in the lot'?
IMHO, I see no reason why one could not have a light on with the sign circuitry, as long as it doesn't overload the circuits.
We have some signage that is tapped of of PL lighting (277V sign & PL Ltg); a 'few' floodlights mounted on a sign structure; albeit another circuit or two. And, yes, in a pinch, circuit capacity permitting, a pole light tapped from the "close" signage.
A few strip malls have a few wallpacks/floods that are powered from the canopy sign feed. (OK, no permit & hence no insp.)