I started decomposing that branch and landing small and simple chunks. I will get to the most important and essential change soon. Hopefully this will all be merged in a day or two.
After a lot of review and redesign, the first version of this feature is in snapd master now:
Among other things this involved coming up with a new way to securely perform bind mounts on directories controlled by the user, which I wrote up here:
The next step is to use this infrastructure to enable access to
xdg-document-portal in the desktop interface, which is a prerequisite to use most of the interesting features of