I guess the main question I here is: Why would someone opperate a massive J6 ship and fillnit with other ships rather than cargo of their own? For the destination it would be hugely disappointing to have such a ship arrive in system only for it to offload a number of tramp traders.
One possibility is that a route is already well-served for conventional cargo shipping and this serves a different market (though whether or not it's viable is another matter).
As for the disappointed systems, perhaps the ship's requirements, its crew, and its passengers all represent other business opportunities for the locals.
I can see the challenges inherent in making this work. Traveller is a game that takes some pride in having a realistic (at least with the parameters of the setting's concepts) and consistent set rules for key elements like space travel, ship construction, and trade. Push things too hard and they don't look right anymore. It becomes a matter of balancing any wonkiness in an idea against anything interesting it brings to the game in terms of plot and setting.
Aside from potential story ideas, something this might bring to the table is a philosophical idea within the setting itself: ordinary people can say, "Yes, I'm part of the Imperium and it matters to me, because I/my family/my business can move around in it within tolerable timeframes."
Another thought, some cost elements could be brought down because you're effectively transporting qualified people: a percentage of your crew can be the very people you're transporting?
Just thinking out loud here.
Regards,
Robin