I was wondering that: it is stated that the harbinger was already used before the deployment of the ursa.
My question is, how would it have been deployed? By capital ships? Transports?
In my headcannon, I imagine Cardinal Spear's Gorgon as the early GTA "big bomber." Like most of my projects, working on the Gorgon didn't get very far. However, the design I had in mind (replacing the Medusa model, of course) consisted of the following:
1. Substantial central fuselage with an engine mounted at the rear. There is a single turret on the upper hull as well as the lower hull. There is a stepped cockpit area, not unlike that of an attack helicopter, though the WSO does not have much of a forward view. There are two bays in the central fuselage, one being the forward-mounted self-defense bay (mounts the Vulcan cannon, rockets, or missiles) and the heavy ordnance bay. The latter is where you'd deploy your huge munitions, or perhaps even kinetic kill vehicles onto a planetary target. This was intended to be a bit like a free-fall bomb bay in terms of its design. For Cardinal Spear in particular, it's the bay from which you'd lay mines, etc. Probably had some sort of fancy name like "Multidirectional Ordnance Projector," or other such jazz.
2. Two stub-mounted engine and launcher pods. Two additional engines, along with the fuselage unit, were to power the Gorgon. These pods also housed a more conventional launcher array as you'd see in other GTA bombers. They also housed the plasma cannons, which are delightfully overpowered in Cardinal Spear.
...Of course, then I'd have to come up with why it disappeared from service (cannon fodder, unreliable, old, etc.). But meh.
Others have already suggested the bomber/strike fighter version of the Apollo or even the Athena - this would have to be done via an external rack. Not sure of the size of the Harbinger in relation to the spacecraft in question, but I hardly think that was a legitimate question for any Volition developer, ever. However, you've got a nice spot between the lower engine nacelles on the Apollo, and the Athena is wide open from below (though ground clearance may be an issue - but again, size... do we even care lol?). My headcannoning would offer that in the era of unshielded fighters, unshielded ordnace which gets shot up tends to explode violently beneath the host craft when the pilot attempts to arm it. It is a rare munition which gives and takes, after all.
The best candidates here, which have already been suggested, are the Fenris or Leviathan, or perhaps even a destroyer in orbit. FS, being fighter-centric, is unfortunately not very kind to the big ships, and their capabilities are often seen as limited at best.