JMO, but I believe that since most commercial sign and parking lot lighting is normally on over three hours, it would be considered a continuous load.
Since these circuits would need to be calculated at 125%, I think the only way to really do that accurately is with individual dedicated circuits having known maximum loads.
I see that 220.14[F] gives a 1200 VA minimum rating for each required branch circuit.
I’m only guessing that "required" would mean, as needed, since 220.14[F] references 600.5[A] and as you already noted, 600.5[A] only states that there must be at least one accessible outlet for sign use at each entrance and that the branch circuit for the one or more outlets be rated at 20A and have no other outlets.
This seems be another one of those cryptic NEC requirements that require additional background information as to the reasoning behind it.
