Snapshots only capture the database schema, so the addtional migration information is not available when comparing snapshots.
The current implementation of Migrations requires a connection to your version control system. To generate a deployment script that includes your migrations you should compare revisions from SQL Source Control that are equivalent to the state of the deployed database.
You might find some of the resources on this page interesting: http://www.red-gate.com/products/sql-de ... tegration/
I'm working on revising our approach to migrations at the moment, to improve branching and support Git and Mercurial. I'd love to get your feedback if you have time - I'll send you an e-mail.