I think people here are implying that this needs to be some sort of desktop native app. But I assume Eric is intending this to be part of WordPress itself (I could be wrong though). So you'd just navigate to your admin panel when offline, and it would still work via the browsers offline caching systems. I've messed around with this stuff in the past, as I was trying to make a plugin which would allow WordPress sites to work even when offline (albeit I wasn't intending to tackle the admin panel), but I gave up due to offline storage being buggy in all the browsers I tested. That was quite a while ago though, so hopefully browsers have caught up a little now. I did have a working prototype of that system running (for the front-end), but it bugged out so many times that it became highly annoying. Sometimes you would navigate to the site and find that half your assets were non-existent, or web pages which had been updated would serve the stale cache despite being refreshed (due to browser bugs, not bugs created by me :P).

EDIT: I should have read Eric's post more thoroughly. He covered this scenario in the comments by saying that it would be fraught with issues due to lack of browser support.