IME, you can't.
[0] https://github.com/vagrant-libvirt/vagrant-libvirt#synced-fo...
I see this as a vagrant issue though. It really falls short of its promise of being a dev environment where you "never say 'it works on my machine' again".
why can't you do it in the user space within those VM's, communicating over a network adapter? If you don't care about putting on a raincoat, going outside to the Internet, and launching it up into the cloud, you can just install dropbox and get a sync'd directory without additional work. Obviously this is extra unnecessary overhead, two times, but for your use case you may not care.
you can alternatively roll your own solution though it might take you an hour or two. think about rsync over a local network adapter.
I realize these are hackish solutions but if it's stupid but it works, it's not stupid.
on the plus side you will control that traffic. having directory syncing that breaks the VM abstraction opens leaks due to potential oversights in how it's coded.
if I were in your predicament I would work around it.
The difference between this proper approach, and what you call "hacks" is minimal, and basically a question of packaging. To be clear, I agree that the VM developer should write and package these utilities for every major guest operating system it supports.
