So I've managed to get some infantry production going and sent a couple of them into a vehicle (AAI Warden, from the mod of the same name), and this worked great, they automatically fire from the vehicle and everything. But is there a way to get them out of the vehicle? They don't seem to be in the inventory, and I can't select them by drag-selecting over the vehicle.

The infantry are not but in the inventory but in the grid of the vehicle, they basically function as equipment.

Ahaa! Thank you, I hadn't thought to look there - guess I would have realized once I unlock modular armor and start putting stuff in.