I've been thinking, and regarding the spiritual requirement for Allomancy, perhaps it's the process of snapping? I believe it says somewhere in Mistborn that one couldn't be an Allomancer until one has 'seen death' or something like that. Maybe it is experiencing such an event that creates a spiritual link?
On that note, has Brandon confirmed/denied/RAFO'd whether snapping is still a requirement in Alloy of Law times, post Harmony? I cannot recall seeing it in any interviews.