Steel lines will run cooler and maintain constant pressure/flow characteristics. With that being said, yes one can run whatever kind of lines he wants and the fluid will get there....
Agreed, plus for OEM it' s more expensive to use molded hose vs steel, which is really what would be required in many instances...
The other thing to consider is longevity, the hose will breakdown over time and start to leak or completely fail. If you do steel lines they will last a lot longer (properly routed and supported) than the hose. In some of our training material they give a life cycle of hydraulic hose, the steel braided, high pressure stuff and it is rated to last "40 quarters" or ten years. That is a much different hose than what you buy for a trans line, that hose only has a useful life of 3-7 years depending on the conditions it's used in.
You can adapt the newer stainless lines found in the J'yards off the late model picks and vans for this too. I did this on mine using a set of lines off a wrecked 92 E150, the bends are pretty close to fitting as they are. Just needed a little tweaking to make em work.
I was thinking about doing that, (not really) thanks for the idea...I know a J"yard that has a few older Fords