Currently you'd get a hefty RP reward by bringing in the cargo truck (online Invasion 0.89, single player Invasion 0.90).
Making it spawn a rare weapon or making a rare weapon available at armory wouldn't be out of the question either, but maybe that's logically odd -- does it imply that your faction rewards you a rare weapon for the delivery or that the rare weapon was in the cargo truck? If latter, maybe it would be better to have the truck feature an item supply point at the back (like an armory (=backpack), potentially with limited and specific supplies), and you could actually access the supplies even before delivering the truck anywhere.
If that way, the truck (or some other truck) could actually feature all sorts of goodies. As 0.90 allows "vehicles" (static ones too) to be created from items, the truck could offer some engineering resources to be deployed like pieces of sandbag walls or other stuff.
Finite armories does bring up the question that does respawning use those resources as well, and if so, do bots use it too, and if so, shouldn't bots also try to search for cargo trucks and deliver them to home? Failing to find them could perhaps easily starve the faction of weapons.. I guess spawning would stop or people would spawn without weapons then..?

It's an interesting concept but I do have concerns the outcome gets out of hand quickly and it becomes a balancing nightmare.