Couldn't you count each belt delivery as a separate adventure? If you do that, and you have infinite belts, each of which grants you infinite experience (necessary in order to ensure you never stop gaining XP, even though the XP from each belt is truncated), and you have infinite people to deliver them to (simply deliver one to one trillionth of the inhabitants of Sigil, thereby ensuring that the odds of any party actually ENCOUNTERING somebody who received one, or of them affecting the setting meaningfully, are close to zero while still producing an infinite number of them), you can still gain infinite levels, right?