OK, I tried all of the suggestions in the thread but noting was an exact blueprint for this situation. But now I have a result that I'm happy with:



The Magenta area was produced from a rail loft. Simple surface-to-surface blending produced creases at the corners. I used several curves produced by putting lines along the surfaces and joining them with blend.

The Magenta form originally went all the way to tho top of the large shape on the left. I messed around with various operation like Boolean Diff and so on to try and cut part of it away. This always ended in something odd happening. I was getting strange surfaces with edges that didn't meet up! The shapes are so close that I think its running into the precision of the underlying math. I also tried the technique of sweeping a circle along the intersection line to make a solid to cut with. No dice.

So I just made a curve in the Right view and projected it onto the surface with Iso Curve and used that for cutting. That works perfectly. With that done I get 3 resulting edges on 3 faces that I join to make a loft curve. Using the technique I detailed in my previous post I chopped and joined the profile of the gray colored form on the right into 3 pieces to match. Then I used Loose Loft to make the Cay form with 1 intermediate control curve. You can see the results of the chop/join in the paths that the Yellow lines take. If you just loft the curves you won't get a result like that.

I still have to do this on the actual model where things are /slightly/ more complicated but I think I have a workflow that will scale to that complexity.

Thanks for all the help everyone!

I've been hacking at this for 3 weeks and I'm super happy I got a solution!!! :)