Odd to hear about liability. Surely it is no different to the liability of the other running gear that was included?
I suspect the chassis was originally sold as a fire sale by BrawnGP (note the headed paper) but the new ownership is going down the same route as McLaren & Ferrari - ie keep everything under tight control.
I'd see more risk for them in selling you the drawing, it would infer design liability. Whereas handing over a bit of scrap metal that has gone through the wars on the track is very much "sold as seen".
Your best bet is to strike up a friendship with an insider, get them interested in your hobby. If you explain that you are wanting to do an authentic restoration then they might get you access to a complete car to take loads of photos and measurements.
Ross has no emotional attachment to this car, but Rubens and Jenson do. JB is rather busy at a rival team, I guess RB is your best bet.
Oh, and contact Top Gear. They have previously taken a "super car" on the track that was built by an amateur in his garden shed, so why not someone with an F1 car in his shed? Also didn't Hammond do a few laps in this model (or similar). Also May would love the attention to detail in getting back on the road. They won't fix it for you, but have connections (and publicity) to help you get a look a working model... or even some drawings.