Ok I just read the Dash overview, and -nowhere- does it mention being able to inject windows into existing apps.
All it talks about is being able to multitask, switch between apps, or run multiple windows and resize them as you see fit inside the new Dash space. There's not a single example or hint of being able to overlay another app into one which is "full screen".
https://www.oculus.com/blog/introducing-rift-core/
What it -will- allow though, is quickly "alt-tabbing" between ED and another App/website while staying within VR. Even being able to display them side-by-side and so on. Each app can be run on its own virtual monitor, which will be hardware-accelerated at the OS level, which actually is a pretty neat thing.
But it doesn't look like the holy grail of being able to inject arbitrary windows into ED will be achieved using this update.
EDIT: PS. if anybody has a better resource (I wasn't able to find one) which shows more in-depth usage and overlay ability, please post links!