First off, thanks to Bill and Todd for the help in locating the shoulder harness mounting points.
I did end up making a parade platform/seat thing. Here's the rundown.
I'm kind of a Unistrut guy for stuff like this, because once your prototype is finished, you're done. I'm sure welding up something from steel or aluminum would work, too. Wood, not so much... unless it was oak.
The design constraints were: no new holes in the car, rock-solid stability, keep load off of bodywork/finish, and make it easily removable. I elected to make a cantilevered support, with two mounting points. The rear mounting point is the shoulder harness one, and the load is down when the parade seat is occupied. The front mounting point is attached to the outer lap belt mounting bolt, and the direction of the load is up at this point. The parade victim sits suspended over the stowed convertible top, without resting on any bodywork.
For mounting points, I used the aforementioned shoulder harness tiedown points, and the outer lap belt tiedown points. I was fortunate to have on hand some 7/16-20 fine thread bolts, with a seatbelt eye. (Not a standard eyebolt...) I replaced the lap belt outer mount with one of these eyebolts on each side, with the eye straight up and down. That allowed me to bolt on a Unistrut fitting to tie in the front support.
The rear mounting point was the harness mount. Had to totally destroy the rubber plugs to gain access, and I had to run the threads, too. Using several Unistrut plates, brackets, and whatnot, I was able to craft a pair of brackets with a 21" front leg, about a 10" rear leg, and a beam across the top of some 28" or so. (I'll grab the actual measurements tomorrow, and edit them in.) The horizontal beam on each side picks up both legs, and also holds the 2x10 SYP seating board.
When installing, go for the back legs first. After they're built up per the picture, they will need to be twisted so that the front of the bracket clears both the door post and the seat. (Attach the horizontal top piece and use it for leverage. Watch the bodywork.) Next, measure up the needed length of the front bracket, cut to length and install. Make sure the horizontal top piece is clear of all bodywork. Also watch that the trunk lid doesn't hit the back of your platform when you raise the lid.
Finally, tighten everything down, install your seating board, and hit the road... slowly. (I ended up providing a couple nylon web loops for the occupants to slip around their non-waving wrist... just in case the clutch cylinder chooses that very moment to crap out.)
Here are some pix. Maybe. The only other thing that was added after pictures is a diagonal brace to reinforce the 'two leg' stance.
I'll update in the next day or two, and try to find someone who took some parade pictures.
Ken
