As an aside, the instigator of this thread is, I believe, labouring under one common misapprehension: Downwash does not primarily increase lift, it primarily reduces drag - hence Wing In Ground Effect designs.
When I was part of a team using Dakotas for spraying oil slicks, we were almost certainly enjoying some of the benefits of downwash, as we were spraying at 25 feet over the water - well within the theoretical half the wingspan height at which it works (we were also enjoying various other aspects of the job too!)
As I remember, the aircraft would become a little more stable down at spray height and, rather than wanting to climb (more lift) it would simply accelerate very very slightly (acceleration normally being pretty slight at the best of times in the dear old gooney bird!). Although, I have to admit I haven't done it for some years, and memory may be playing optimistic tricks on me...
These links have something on downwash:
http://www.pilotsweb.com/principle/liftdrag
http://142.26.194.131/aerodynamics1/Drag
I've also flown the 747, and I must admit I had forgotten about the extra pull late in the rotate.
Cheers