Let me state that I have neither the large basket nor the Ballard Cargo bags. However, I have both a basket and side bags installed on my RW4.
I made a wooden 'I' that bolts on, replacing one of the wooden slabs on the back of the bicycle. I then zip-tied a wire basket on top of this wooden structure. That provides me with enough lift to be able to attach a side bag to the metal tubing of the rack in back, as well as hook some luggage straps to compress the bag.
Finding something to act as a spacer between the basket and the attachment point on the rear rack would probably work.
I agree with you that Rad didn't really figure these obvious things out. For instance, why isn't the headlight compatible with a front rack? Doesn't anyone at Rad actually use a RW4 to haul a bunch of groceries and other things around?