I'm familiar with "Google Apps Directory Sync" and "Google Apps Password Sync", but both only work one way - with the Active Directory being the authoritative source and the Google Apps consumes existing accounts.
My organization is heavily invested in Google Apps and I manage all the data and account there. We are looking to add Active Directory to the office network to allow office users (which does not include all of the organization users) to have unified workstation login and file server access (file servers are running FreeNAS), using the same credentials they use with the Google Apps domain (also SSO would be awesome, but not a requirement).
If there a way to keep Google Apps as the authoritative source of account and sync into Active Directory?
There aren't any Google-provided tools which do this. It would be possible build your own Google-to-AD sync for almost everything except the Google Apps passwords. But if you do SSO that would be less of a problem. Check out Google Apps Manager, or the Google API Client Libraries.
I think the dearth of such tools is because most companies still rely upon their internal data sources. For instance, the HR or Business departments' applications. AD would sync from these internal sources, then data gets pushed out to other applications such as Google Apps.