First, as Jared already pointed out, this is not possible without a "local copy" – i.e. without the data being present in the device's file system. Second, it won't work without root access on the device – as the directory in question (/data/data) is not accessible for a "normal user" (which includes apps: each app can only access its own sub-directory here).
Let's assume you're having root access on your device, this would still not be easy. Theoretical approaches:
- using an app like FolderSync to synchronize your local
/data/data with a counterpart in your "personal cloud". Might work somehow.
- copying the entire
/data/data to your personal cloud and remote-mounting it (e.g. using SSHFS with SSHFSAndroid or some other remote mounting). Again, might work – but if your network connection drops, apps might crash, plus you may risk data inconsistencies and data loss.
Apart from the risks named, there are more concerns:
- privacy and data security. Might not be a concern if you're just doing that within your own (home) network, but it at least introduces another attack vector.
- you might be tempted using such a "network share" with multiple devices simultaneously. Don't. That would be the best way to corrupt your data, have your apps crashing, and more.
- performance: if all your app data have to be read from/written to the network each time you access them, it would slow down things noticeable. And I mean, real noticeable here – not just some digits somewhere after the decimal point.
- file permissions and ownership must be maintained, which might prove tricky (depending on the remote storage).
If it were such an easy thing to do, it would have been done already – especially considering Google wants us to put all our data into their cloud. There are good reasons speaking against that (and I'm pretty sure this answer is far from covering them all). So while this might be technically possible, it's definitely not recommendable.
