I've take a quick look at this and I believe this is something that could be modded in one of two ways.
First, the caravan unit is defined in the CoreVehicles.xml file. It's possible that you might be able to make this a upgradable unit. I'd have to play around with it. Idealy you would want it to have a group of guards, rather than one guard in a tactical battle.
The second would be to add a new Caravan unit, by adding a new Technology. The techs for the kingdoms are in the Techs_Amarian.xml file, but you would have to make sure that Kingdoms also had a tech upgrade path. You would then just define a new Caravan unit with a Attack and Defense rating.
If I have some time this weekend, I'll play around and see if I can come up with a method to make this work.