>1) I'd make it have an earthbinding affect that disallows
>flight until the object is removed, either manually or via the
>idea in #3.

This doesn't make sense. An assassin having to step out of the shadows for a round to quaff a temporary flight potion whose effect can be dispelled...that's enough of a deterrent right there, compared to arial flight. I'd do your #3 differently, though. See below.

>2) No wingsweep.

Agreed.

>3) Make it such that when they spread their wings with the
>object on it falls to the ground.

Don't like this. I'd say make them have to remove it to spread their wings, which means, among other things, they would need to be able to see. I might also associate some lag with removing a cape, since you have to extract your wings from under this cape, but then there's no lag associated with removing a set of full platemail, so maybe none is needed here either.