Remote Access/Editing files from Android without rooting on any other remote platform
I guess the part rendering solutions close to imposible is without writing the file to the Nexus 7 local storage at any point from opening to saving and closing. If that should also exclude automatically created temp files, you're very much out of luck.

Additionally: and using any application on the Nexus 7 Yo. That means either all apps have to support Samba/CIFS/whatever network protocol to be used – or you need to mount that network drive locally. This brings it down to apps like CifsManager (last release: 4/2011) or Mount Manager (requires root; I would wonder if that doesn't apply to CifsManager as well). Moreover, on your remote device you will need the counterpart providing the network drive, of course. No problem on the PC, where it comes with the OS. For your other Android device, as pointed out in my comment on your question, that could e.g. be Samba Server.