Internet rates don't match a good local network so you won't be streaming bluray. Also it's a security risk. You Modem/router firewall is there to prevent this happening. http://www.zeitoun.net/articles/samba-over-internet/start
That link dosn't mention that that you use you Internet IP to gain access. Unless you pay extra for a fixed address that can change every time your modem/router boots.
So having set up port forwarding on the "server" med network, you would set up a Nas shortcut on the client Med using the "server" networks internet IP, the one buried in the modem/router.
Another issue will be buffering, or lack of it, using the normal lan based sharing system. At some point in the near future an IPTV client will be added to the Med which should work with any internet server supporting IPTV streaming. That client will have to have buffering to work adequately. Unfortuntely that dosn't mean the Med's current Nas function. Adding an IPTV server function to augment the Med's NAS function would allow multicast streaming from one Med to multiple Meds equipped with the IPTV client.