No, there are Omnis present in most of the second-line Jag units listed in The Dragon Roars scenarios.
In most?
Scenario 1 shows frontline unit.
Scenario 2 shows frontline unit.
Scenario 5 shows frontline unit.
Scenario 6 shows frontline unit.
Scenario 7 shows frontline unit put in second-line Galaxy (Jaguar Regulars). Eight of fifteen 'Mechs are Omnis.
Scenario 8 shows frontline unit.
Scenario 9 shows frontline unit.
Scenario 10 shows frontline unit put in second-line Galaxy (Jaguar Regulars). All Omnis.
Scenario 11 shows frontline unit put in second-line Galaxy (Jaguar Regulars). Four of five 'Mechs are Omnis. By the way, this scenario makes Victor Steiner-Davion a pilot with Gunnery "0" if you use the special rule offered.
Scenario 12 shows frontline unit put in second-line Galaxy (Jaguar Regulars). Five of ten 'Mechs are Omnis.
Battleforce scenario shows frontline unit.
In whole scenario book we don't get a glimpse how typical Provisional Garrison Cluster may look like.
OmniMechs are present in Galaxy Command Trinaries or Keshiks, but usually not among common second-line warriors filling most of PGCs.
The RAT listed in HTP: Luzerne also features OmniMechs, at least half of the selection for all weight classes. The one Front-line unit in the "book" (out of three Jag commands) has the option to trade out BattleMech selections for equivalent-weighing Omnis. I'd therefore say using the RATs from Historical Turning Point: Luzerne would be the best ones to use.
In
HTP: Luzerne player may choose between one Galaxy Command Trinary (which can hardly be considered common), one frontline Cluster (which is not in scope of this thread) and one PGC (which is fluffed as as one of the best second-line forces CSJ had to offer, and which could easily pass as frontline unit). That's hardly a typical situation and hardly a typical Smoke Jaguar garrison.
Worth noting the Jaguars were the ones building Stone Rhinos (Behemoth) so there's a very probable Second Line big boy for the heavier units. Heck they were the ones who developed it from the failed Matar prototoype (AKA Amaris's folly)
And that's why I allocated two slots in my Jaguar RAT for them.