After a (quite challenging!) 6 hour campaign as Senones I got Brennus to Roma and sacked it. No sword nor "weighted scale" (inv_weighted_scale) gained nor anything else. You only get some payment reward if you sack Roma during the chapter 4 or 5 I think, it is just a chapter objective (invasion.pack/campaigns/main_invasion/missions.txt.)
There actually many events/dilemmas, for example one is about... (minor spoilers ahead)
But I see that these ancillaries used by the events are not used in export_ancillaries.lua. So the most reasonable conclusion is that ancillaries with zero chance are just unfinished and probably not received by other means.
There are a few other unfinished ancillaries for Rise of the Republic like "Illyrian Captain" (for pirates), "Illyrian Traitor", "Horned Helmet" (for Iolei), "Weighted Scale" (For Rome. And provides a bonus when looting settlements. Shouldn't this be for Senones?), "Dionysius Poetry" (for Syracuse), "Statue of Juno" (For Rome, after winning a battle in Vei from what I see)
I want to add them in the game as it was intended but the conditions for their appearance are a little bit weird. Is it appropriate to take initiative and create new, proper/logical/historical conditions for them or would it be too much creativity for a "bugfixing" mod?
For example I would like a general/admiral to receive "Illyrian Traitor" after a victory against the Illyrian Pirates (it provides bonus attack against Illyrian Pirates) but right now the conditions for this ancillary show that it will be gained by the Pirates, not other factions.
In the meantime, as I await advice, I will skip these for now and continue to the rest problematic ancillaries that have more obvious solutions (like wrong building ids, wrong availabilities, etc) and report back again.