I don't know the exact configuration of the wagons, but I reckon there are two issues with any seat belt:
1. Where to mount the top anchor.
2. Where to mount the retractor.
For the front of a wagon, I am assuming the top anchor can only go on the post. And I would weld in a plate to the inside rather than drill a bold through from the outside. For the retractor - these can mount off or close to the floor by the post or maybe fitted in behind upholstery behind?
For the rear, I am not too sure where the top anchor would go? Is there another post back there? And I guess the retractor would fit to the rear floor, maybe with a bit of support.