You would be suprised at what can be done with handforming. The kind of die you are talking about, generally would not be made of steel plate, but of kirksite, and would be polished to a mirror like finish. Any imperfections in the die surface would be imprinted on the part during the forming process. Frames and bulkheads would not be formed in pressbrake dies, unless they were straightline pieces. Most are formed either in drop hammers, rubber forming or by hydroforming, so contours and curves are not big deals.
There are various books and guides out there, but for some quick, but general knowledge, try looking through youtube. Here's a nice one that forms a fairly straight line rib, but has a contour change in one face: http://www.youtube.com/watch?v=5loIfMfOV5Y .
About the only way you would be able to really make your money back on building some hard tooling, is to go through the process and get a PMA for the particular part and produce them in bulk. Designing a die doesn't have to be very difficult, but there are quite a few parameters that you have to have onhand before you start. These include forming method, press size, tonnage required to for the part in the material condition that you choose, heat treat requirements and so on. There are numerous volumes out there on die design that can be referenced. Additionally, you are going to have the problem that if your die is for a drop hammer, anyone that wants to use it will also have to have a drop hammer....likewise if it's a hydroform die. I can guarantee that there are very few repair shops out there that have of the heavy forming capabilities. Offhand, I can think of less than a dozen restoration shops that have any of the processes in-house.
The last thing to consider also is just flat out the cost of the die. Although I haven't been around any die fabrication in a lot of years, I can tell you that it gets pricey real quick. If the part can't be contained within an area of about the size of a #10 envelope, you are going to be above a couple of thousand bucks.
If this is a problem part for all of the Lakes out there, it might just be better for the owners to all get together and bite the cost and have a die made and get the PMA for the part and split the costs between everyone.