While I think that the ideal solution would be to extend Microdata Syntax to allow for multiple types, this solution will take time as it necessarily entails a standardization process. šAs an active member of a Standards Body, I certainly value and sympathize with the need for a deliberative standards process.
That being said, I feel that the need for web publishers to associate multiple types with their metadata is sufficiently urgent that the sponsors of schema.org would be completely justified in introducing an 'additionalType' property. šFar better, in my view, to have a good solution tomorrow than a perfect solution at some indeterminate later date.
All the best,
Lead Architect, Semantic Plaforms
The New York Times Company