Hi,
I don't think there is a database on car payloads.
When I worked for Citroen pre 2003 Multispace 2.0 hdi had a payload of 450kgs which I got from a phone call to Citroen Slough on behalf of one of my customers. The panel vans were 600 and 800 kgs payloads, depending on model.
I was told the reason why they were less was that the Multispace was a people carrier and needed to be more softly sprung.
I would assume by that reply that most people carriers based on panel vans would have a lower payload that the panel van.
Spruce