Hi,
No, there’s no way to do this, and nor will there be, I’m afraid, for good reasons. Scrivener is a Cocoa document-based application, and such programs are built on the paradigm of one document (project) per window. It would need a completely different infrastructure to support multiple projects tabbed in one window (i.e. a complete rewrite from the ground up of the core project handling code), and in doing so it would be going against the standards laid down for Mac applications by Apple. (Excel doesn’t have different projects open in different tabs but sheets that are part of the same document, for instance.)
Hope that makes sense!
All the best,
Keith