I found if I place a storage area over the items, they appear in the setup screen for a caravan. It allows the colonists to load it without taking a diplomacy hit. Once the items leave the map they are no longer restricted.

How do you make the button to reform caravan available? My colonists were attacking the pirate city, killed almost everyone (only prisoners from friendly factions left) and I still couldn't press the button because "there are enemies on the map left". So I had to just leave the city without much looting through the edge of the map.

My understanding is that enemy settlements in vanilla RimWorld also have the restriction on reforming caravans during colonist raids. If enough people would like this feature, I could perhaps add it as an option in the mod settings.

Yeah, being able to reform your caravan whenever would be nice as an option. That'd give you an option to do a snatch and grab or whatever.