Some ideas-
In the Sienna, items stored in the "well" behind the 3rd row seat are already somewhat secure, much like they are in the Odyssey or MPV. The well itself is part of the chassis, and items are restricted from moving forward by the frame. The back of the 3rd row seat provides another barrier from forward motion of items stored behind it. It would take a pretty severe multiple impact and/or rollover crash to fling items around the cabin from the back.
I think standard cargo net or bungee cords (using cargo or tether anchors) should secure most items behind the 3rd row very effectively. Groceries on grocery hooks and a bungee or two on a stroller should be pretty effective for most crashes.
Heavy cargo is otherwise pretty difficult to secure in many vehicles. Bungee cords and standard nets will help, but may not be enough. I don't know of any solutions for very heavy objects in other vans, wagons, SUVs, or if the 3rd row is folded down. Some auto makers might have a cage or heavy duty screen intended for dogs or other cargo, but that would vary by company.
My stroller and cargo bins sit nicely in the well behind the third row, as does the stroller. I do use a bungee attached to a tether anchor and the leg of the third row seat to keep them down.
I hope that helps a little!