In the end I just built a slightly higher-res (810x486) version from the last frame and a stretched first frame from that sequence.There was no "lens" used - just a bit of guesswork and approximation. Each bit of the background is zoomed a different amount based on the sine of it's distance from center (multiplied by a different factor for left and right), plus a vertical shift to create the downward motion. The vane is zoomed with a linear function, which is equal to the background at the left and rises to the right, plus the same vertical shift. I was amazed it came out so well, and I think it vastly improves the shot of what was otherwise a very static painting.
The guys at LFL would probably look at that and think "wish we could have done that!"
They might wish they'd thought of it - they could definitely do it
DE