When I carpeted mine I found lots of hexagonal holes stamped in the interior metal work, behind the panels. Also found hexagonal rivnuts on ebay. I think these would be fine for securing cupboards, ply panels, hanging curtains, coat hooks etc but I wouldn't strap an MTB down with them or secure several sheets of plasterboard.
When I removed the row 3 seats I found some 7/16 eye bolts to fill the upper seat belt holes (used in race cars to attach harnesses). Now these would take a lot of weight! If you didn't have seat belts you could always get the threaded plates for fitting belts in vans.
Ian