You are correct the auto-sync only works for content related items. From a security standpoint, you should NOT have user accounts in non-production systems, it is NOT a good practice and can open your systems up to major security risks. This is true with any site, in any CMS.
To answer your question, you can create a custom scheduled task to sync objects on a scheduled basis.
You're essentially rewriting the process already in place for pages. You can use the API to access the staging logs, then look for specific object types and sync them over.