... flew across the Ring to the exact dead center, then dropped? Would he land on the top of the Spire? (I presume that even if it worked, said character would be falling fast enough to be squashed flat on impact! Hard way to achieve immortal fame...) Or would he be shunted to the side and forced to fall down the side of the Spire (thus missing the top)?
What would happen if a flying character in Sigil...
The way I'd interpret it would be that the exact center would have no gravity, but that anything that drifts off-center falls towards the inner surface of Sigil. If you drifted out of the plane of the ring (plane in a geometrical sense here) you'd be pulled back to the center and might even bounce back and forth slowly.
It would be an interesting quirk to have the center actually be a stable point in all directions for whatever reason, such that random crud collects there over time until it reaches a sufficient size, at which point it satisfies the conditions as a portal key for a Sigil-sized Radiance portal, thus connecting somehow to the timely varying lighting conditions of Sigil, but thats perhaps a bit much.
The Spire isn't visible or accessible from Sigil. Effectively, you're infinitely far above it.
I propose that the Spire doesn't have a "top" to be struck or missed.
Also, Rip is right. The Spire and a ring that looks like Sigil are only really related from the Outlands. From Sigil there's no observational relation to any other plane, Outlands or otherwise.
Pants of the North!
Infinite drop. He wouldn't hit the top of the spire at all.