After seven years and 30k miles I can say this works. Happy to share the detail with those genuinely interested.
Situation - No end of motorhome rack manufacturers build them to carry bikes and scooters but only to take up to 125kg's or thereabouts. We wanted to extend our riding season by going south but bit too old for the distance (and the best of your tyres have gone by you get to the med). Don't fancy the idea of towing all that way and the best places to stay are often tucked away in narrow lanes and villages where reversing can be a problem with short bike length trailers.
Answer - Build my own rack that extended the chassis. Rather than using the tow ball mounting so being restricted to the towing nose weight of the van, this let me carry loads up to the limits of my vans tyres, suspension and the maximum allowed gross vehicle weight.