I think this warrants it's own thread :)
With a precise jump from the spot I show in the video, you can trigger flymode indoors consistently. This means we can trigger flymode at the start on the way down, and use bird for rest of the archives :)
I think this is what KimInTheAttic meant in her thread.
I am not sure on the mechanics, other than it seems to require either a certain height, or passing over very close to ground, but then missing it and landing somewhere else. As you can see in the video, I just about miss the ground after the jump, which I believe triggers the glitch.
tl;dr Can activate fly indoors consistently, the strat seems to be to jump into freefall, get very close to the ground, then miss it and it should trigger flymode.
This was quick to find :)
I'm very sure there is one on sanctuary. When I finish the game I'll go back and check. If we can flymode, I think we can skip the entire level, nearly.