How do I distribute Record Type Assignments when packaging Permission Sets?

Bah. You can't. Salesforce have now documented their way out of this one ;-)

Best Practices

If users need access to apps, tabs, page layouts, and record types, don't use permission sets as the sole permission-granting model for your app. Assigned apps and tab settings are available in the permission set user interface, but they aren’t included in permission set package components.

salesforce.com/us/developer/docs/packagingGuide/Content/packaging_perm_sets_profile_setttings

So: Record Types are destined for the domain of the installor, not the packagor. The the same way as Profiles are owned by each org administrator and should be extended by Permission Sets.