I recently acquired a shiny old Viano for my transport needs.
I want to keep the 2 mid-seats for the odd occasion when I need to take the kiddies as well as my gear (e.g. to Sun & Sea @ Jarvis Bay in Oct!).
Before building anything substantial, rack wise, I wanted to test a load out and see how it felt.
Seats out, except 2 folded down:
3 Booms fit under the seat
Wetsuit box fits under the board (timber bracket comes later)... room for a big box before the boot shuts too!
Foil board horizontal... 60L, 90L and 110L speed/slalom boards fit vertical down the side, resting on masts all in bags.

Hanging sails didn't feel safe, so I gave up on that idea!
Fully loaded with just 1 piece of timber for a rack.
4x Boards
6x Sails (10m to come)
5x Masts (550 to come... should fit between seats too)
3x Booms
Foils, Fin Boxes, Wetsuits, harnesses etc.
After a few trials, I plan to have the nose of the board raised and fit some gear under the foil-board, above the black box and seat. Later.
The problem with passenger fans is there are few exposed mounting points.
The alternative would be to build a rack in such a way as the seats can be removed and installed without rebuilding the whole interior... we will see.
Now all I need is some wind :)