Just looking through the "Making of FJ" video and discovered some things I am not sure have been discussed. First, It appears we will be flying directly at the quidditch stands at some point, perhaps in a near collision. (conjecture on my part)
Second, while I am not in the movie making biz, I have NEVER seen a composite shot against anything but a green screen or blue screen. Never a black screen.
We kinda know that the the following was shot for the Musion effect in Dumbledore's Office. Note that the background is black as would be for some Musion effect shots. (not all, but some).
So, we have two other hints of Musion being used within the ride itelf. Note that both of these are also shot against black screens.
Just thought I would throw it out there for discussion.