Yes, in particular we could look at sync between your browser and some box ("NAS") in your house. Instead of running ownCloud or Kinto on a server, you could be running it on your "FoxBox" inside your house (I think FreedomBox already support running ownCloud and/or remoteStorage servers from your house).
The difference then is that your storage server doesn't have a public IP address, so this involves punching a hole into your ISP's firewall, and inventing a way to discover your storage server if it's not on a public IP address.
I'll be helping with Shweta with her work on Kinto discovery (we were considering something like /.well-known/storage), and I have heard positive stories about using Tor hidden services for this. Definitely worth some exploration! :)
Having said that, it's not much different whether you store JSON data or media files on a server box in your house, so maybe we can decouple that conversation from media files per se.
On a separate note, if the plan to port apps like Music, Camera and Gallery to proper cross-browser web apps (removing their strict dependency on DeviceStorage), then cross-origin sync to Dropbox/GoogleDrive will become all the more important, because once you run them in a non-B2G browser, these apps will be useless without access to where the user wants to store their photos and mp3 files.
Cheers,