There's not a lot of flex between the cab and the tub and when they let go, they usually come apart unless it's a body mount that's collapsed.
I'd check the max load for the roof of the canopy. Not all of 'em can take large amounts of weight. The dynamic loading of the D40 cab's roof is 95kg - that means while the vehicle is moving, the most you can put up there (including racks, brackets, baskets & contents) is 95kg. The static load is higher. I don't know if the D22 has a different loading - I'd assume it would be similar.