We use piano hinge for some of our custom cabinet applications (mainly trick window seats), and the main problem with them is the lack of adjustability. You really have to nail their position the first time, otherwise it's extremely difficult to make them work.
The other problem with them is that if there is any kind of bend or warp in either of the two materials being hinged together, there will be some binding when they are opened or closed. In our applications, the materials are usually flexible enough to flex with the hinge and work, but in welding these to steel I'm not sure you'll have that same flexibility. In other words, these will have to be attached to the vehicle and door dead-nuts straight.
You ought to figure out a Delorean or Lambo opening system