Ability to specify which synced folder to rsync-back

[Idealien]

I suspect some of this may depend on functionality missing from vagrant synced_folders, like an ID to reference for which synced folder, but it would be great to be able to do an rsync-back on single folder instead of all that are defined in vagrant file.

The use case (for me) is having 5 or 6 local WP dev sites running within one VM at varying states of completeness. I use vagrant-back when doing automatic updates from inside the VM back to my host, and rsync-auto when doing incremental dev. There's the performance aspect to updating N synced_folders, but more important is the ability to control which / when to sync.